Nusa Penida is a popular day trip destination for dive operators from Bali. Conditions are "challenging" here, with strong currents and cold (for the tropics) water. People are willing to put up with those less than ideal conditions for the chance to see a Mola Mola - the world's largest bony fish, and probably the funniest looking one as well.
Nusa Penida is the largest of three islands. The island closest to Bali, Lembongan, is a very popular day trip destination for people looking for good snorkeling and water sports opportunities. In recent years, some hotels and resorts have been built on Lembongan, but it doesn't appear that any of these have dive operators.
There are several sites around all three islands. I've highlighted three of the main areas that are the most popular with dive operators from Bali.