Bali has many great beaches - good for swimming, surfing, or just lounging on the sand. Some hotels and villages provide daily clean up and grooming of beaches. Also, the quality of a beach can change drastically between seasons, depending on the location of the beach and the direction it faces.
Bali's beaches are best during the dry season, from about April to October. During this period at beaches on the western coastal areas, such as Kuta, Seminyak, Legian, Cangu and Jimbaran, erosion is minimal and the lack of wind or offshore winds keep the beaches clean and relatively calm. During the wet season, onshore winds deposits flotsam and garbage on the west facing beaches and constant clean up is needed. In addition heavy rains can deposit runoff around the beaches, particularly around run-off streams that flow into the ocean.
Nusa Dua and Sanur beaches are less affected by seasonality, although the beach strip from the Melia Hotel to Nusa Dus Beach Hotel experiences seasonal erosion in the winter, resulting in a very narrow strip of sand. The sand returns for the summer months. 
The west facing beach from Cangu to Kuta can have rough surf and is known to have many rip currents and strong undertow. Safe swim areas are marked by flags and lifeguards are posted in some places. 
Both Mengiat at Nusa Dua and Sanur areas are nice because there is an offshore reef that keeps heavy surf away from the beach. Occasional rough surf is the down side of the Kuta/Legian/Seminyak beach.

Here is a subjective Bali Best Beach list:
1. Nusa Dua from Hyatt to St Regis/Geger Temple. Known as Mengiat Beach and Geger Beach. The most beautiful blue water, pristine white sand, minimal vendors. You can use this beach anytime (without staying at one of the hotels) at the Yasa Segara village or Geger village beach cafes, which are open to the public. The cafes rent beach chairs for about 20k rupiah per day and food/beverage is available from the cafe. There is also a parking lot. Geger is one of the few beaches at which topless sunbathing is allowed.
Another beach to visit is Nusa Dua Beach between Grand Hyatt and Melia Hotel. This quiet beach, hidden in nusa dua area, really a paradise at Bali. White sand. Less crowded. The sea didn't have big wave, just small ripples. But much much better and less dirty compare with Kuta. On the beach side, you can find one restaurants complex that called The Bay Bali. It presenting the specialties from the local traditional Balinese food to international culinary such as Bebek Bengil, Thai food, Chinese food, etc.

Beach by Hyatt Nusa Dua
Mengiat Beach, Yasa Segara cafe area
Mengiat Beach
Mengiat Beach, Yasa Segara cafe area
 
Geger Beach, from Geger Cafe to Geger Temple. This part of the beach is used by seasonal seaweed farmers. At the far end against the hill is the Nusa Dua Beach Cafe, which also has a few rental beach chairs. (June 2009)

Beach club at St. Regis Hotel, Geger Beach

Geger Beach, protected swimming area in front of St Regis Hotel

Keeping it clean on Geger Beach

Mengiat Beach in front of Ayodya Hotel (ex Hilton)

Lifeguard on Duty, Mengiat Beach

The beautiful sands of Mengiat Beach, looking north towards the Hyatt and Inna Hotels.
NEW INFORMATION (Apr 09)- the beach club at Geger has expanded with a new beer bar, currency exchange and cafe on the Geger side of the St Regis.

Sunrise at Nusa Dua Beach
 Sunrise at Nusa Dua Beach (The Bay Bali - between Grand Hyatt and Melia Hotel Nusa Dua)
Nusa Dua Beach
Nusa Dua Beach (The Bay Bali)
bird view
 Nusa Dua Beach (bird view)

2. Jimbaran. Waves and white sand. Beach is accessible from the Jimbaran barbecue restaurants, but is really best when staying at a beach front resort. Most of the beach is not really suitable for use unless staying in one of the local hotels. At the south end of the beach is the Four Seasons, with a private beach club and restaurant (PJ's - recommended). The beach directly in front of the Four Seasons is rocky and rough, not really suitable for swimming. The beach is more inviting 50 meters away towards the beach cafes.
The barbecue restaurants are great at night for fresh fish, Indonesian style, with toes in sand. During the day the cafes serve drinks and simple Indonesian meals. 
The Intercontinental Hotel grooms its beach with a tractor daily and puts out a few beach chairs out for hotel guests. The beach is spotlessly clean and free of any flotsam and garbage.
The beach in front of the Puri Bali and Keraton hotels is similarly groomed and cleaned daily. The beach in front of the Puri Bali is a particularly fine beach, with the hotel providing comfortable chairs and a simple beach cafe.
The beach from the Keraton Hotel north to the airport boundary is know as Kedonganan beach is the the territory of a fishing village, a few fresh fish restaurants, and the local wholesale fish market. Kedonganan beach is wide and sandy, but as a working beach is not suitable for relaxation. It provides a fascinating site in the morning when the fisherman land their catch and the market is at its busiest. 
North of Kedonganan is Kelan Beach. Just walk on the beach till you have passed the fish market, now you are in Kelan. Also Kelan Beach got a few restaurants that serve drinks and meals while you have your toes in the sand. The beach is wide and sandy and is cleaned daily. Kelan Beach is overlooking whole Jimbaran Bay and got the colorful fishing boats in front.

Meticulously groomed beach in front of the Intercontinental Hotel

Intercontinental Hotel beach, Jimbaran, looking towards the Four Season Hotel Jimbaran

 Jimbaran Beach, looking from the Muaya fish cafes towards the Four Seasons Cafe

Jimbaran Beach at the Puri Bali Jimbaran Hotel


Jimbaran Four Seasons Hotel beach front - a little rocky and hard to swim. No worries, a clean sandy beach is just 50 meters away.

The fishing fleet at early morning, Kedonganan Beach, Jimbaran

Fish doesn't get any fresher than this - Kedonganan Beach, Jimbaran
Kelan Beach

Kelan Beach is overlooking whole Jimbaran Bay and got the colorful fishing boats in front.

3. Sanur area. Favorite of many visitors for its old time flavor and local feeling. There is a 7km paved walkway along the beachfront which is great for early morning walks. Excellent spot to take sunrise photos. Numerous breakwaters with shelters have been placed along the beach to protect the beach from erosion. The area around the Bali Hyatt seems to be the nicest white sand beach. This was the first developed resort area on Bali and is very popular.
Beach at Hyatt Sanur     
Beach at Hyatt Sanur
Beach at Sanur
 Beach at Sanur
Beach at Mercure Resort, Sanur
 Sanur Beach

4. Kuta/Legian/Seminyak/Kerobokan/Batu Belig. Nightlife, sunset and beach. Beach is not as clean as Nusa Dua and most areas of Jimbaran and the water is rougher.  Lately, clean up crews have been noticed in the morning. Close to great restaurants and night life. Nightly sunset is a plus and many beach front resorts and restaurants have beach side lounges to enjoy this scenery.
Legian Beach
Legian Beach

Legian Beach
 Legian Beach rental beach chairs
Legian Beach
Entrance to Legian Beach, next to Anatara Hotel
 Legian Beach
Legian Beach

Beach around "Kerobokan", but most know the area as Seminyak. Adminstrative areas on Bali are gerrymandered so sometimes the official administrative name does not match the place name most visitors are aware of. 

 Beach area around Batu Belig
 
Approved swimming zone in front of the Grand Balisari Suites Hotel, near Batu Belig

Stream empties into ocean near Seminyak/Batu Belig. During wet season, the stream is a major river!

5. North Nusa Dua area, from Melia Hotel to Novohotel Tanjung Benoa
6. "Echo Beach" Cangu
Increased in popularity recently due to the vibe from the restaurants built on a cement seawall facing the ocean. Echo beach really isn't suitable as a swimming or relaxation beach, but has a solid reputation as a surf beach. The beach is a bit rocky and the sand is blackish and the seawall on which the restaurants are built is not attractive. The surfing attracts a crowd of surfers and watchers, and it is enjoyable to watch the action from the seawall while enjoying the cuisine from the The Beach House or other warungs. The road to Echo beach has become very pitted and is in need of improvement.

North side of Echo Beach, a bit rocky

Great surf break, just in front of the seawall and cafes

View of Echo beach, looking north towards the seawall
Other beaches of interest:

1. (NEW INFORMATION Apr 09) "Dreamland", now fully developed with a Best Western Hotel. The old surf beach is gone, replaced with a 15,000 rp parking lot. Apparently construction on another hotel, the Raffles is on hold due to the global recession. Avoid.
Dreamland beach resort



2. Next to Dreamland is Balangan Beach. Popular with surfers and not as well known as Dreamland. The feeling of this beach is almost Hawaiian. There are a couple of bungalow style accommodations and most of the surf warungs from Dreamland have moved here. Almost no tourists. Access is a bit of a pain from the Nirmala Store intersection on Jl Uluwatu, but the road is paved.
A bucolic scene at Balangan Beach
A bucolic scene at Balangan Beach.
3. "Kutuh Beach" access from the village of Kutuh through the rock quarries in central Bukit. Has seaweed farmers, white sand, privacy, and no tourists. Really different. No accommodations and no tourists. This is way Bali was 50 years ago.

4. "Green Bowl" accessible from the long stairway at the temple next to the closed Bali Cliff hotel. Surfer beach, caves at the bottom of the stairs and monkeys living around near here. Great scenery and adventure. No accommodations. Really a surf spot.

Stay away from any black sand beach. They feel dirty and the black sand absorbs heat, so its hard on the feet.
Candi Dasa
Pasir Putih (White Sand Beach), about 15 minutes east of Candi Dasa.  Not well signposted but worth the effort.  There are several warungs, all with sunbeds at the eastern end.  Good snorkelling at the western end.
PADANG BAI
The main beach in the harbour is full of small fishermen boats and not really suitable for sunbathing although the snorkeling in the bay is very good, just be careful of the dive boats coming in and out.
1. Blue Lagoon
East of the harbour, up and over the hill is Blue Lagoon beach, small and friendly, with a couple of warungs for drinks and food, and some limited sunbeds.  The snorkeling here is outstanding, straight from the beach.  Currents can be strong in the afternoon, so if there are no dive boats in the bay, this maybe because the current is too strong.
2. Bias Tugel (also known as white sand beach)
West of the harbour (turn left by the police station and walk up the hill) is Bias Tugel, which is the locals beach.  Much bigger than Blue Lagoon but without sunbeds, there are a couple of great little warungs here.  Beautiful views over to Nusa Lembongan and Nusa Penida, and if you snorkel, you may find turtles in this area.
NUSA LEMBONGAN
 Lembongan Beaches
1. Mushroom Beach : Considered one of the best beaches on Nusa Lembongan; swimming, white sand, snorkeling, fishing, Banana Boat, Para sailing..
2. Dream Beach : A beautiful, quiet and private beach.
3. Tamarind Beach : Area of seaweed farming but provides access for surfing spots
4. Song lambung Beach / Lembongan Cliff beach : beautiful beach, very nice for swimming, relaxing, Snorkeling and surfing.