I believe the two major jumping-off points for diving are on the east coast around the airport (forget what that harbor is called) and a couple hours up the east coast in a cove around the village of Pedangbai. From the latter, day trips and more go to the Gili Islands, a famed diving spot. There are a couple of hotels of decent quality but not too expensive in the town of Candi Dasa, a little further up the coast from Pedangbai.