The big diving spots is the wreck of the USAT Liberty, which is spectacular but eerie, encrusted with marine flora and inhabited by thousands of tropical fish. There's also the Tulamben Wall, a 60m drop-off into Lombok Strait. The wreck is a very popular dive, so to avoid other groups you can stay at Tulamben. There are may dive operators and hotels, dive early or late in the day.
The island is in the Taman Nasional Bali Barat, accessible by boat from Labuhan Lalang. There are lot of sponges and fish, great visibility and spectacular drop-off. It's regarded as the best diving on Bali.
You can dive from the beach or get a canoe to go to the best site, especially the Blue Lagoon. There are lot of fish and colder water than south Bali.
Sanur's reef is colourful and has lots of tropical fish which can be seen at depths of less than 12m.
Nusa Penida
There are dive spots all around Nusa Penida and Nusa Lembongan. Dives around Nusa Penida are more demanding with big swells, strong and fickle currents and cold water. The amoun of large marine life, including manta rays, sharks and turtles is impressive.