Sumbawa is a large, convoluted island between Lombok and the Komodo Island group. The diving around Sumbawa is absolutely fantastic, and certainly ranks among some of the best, with a wide variety of dives, from muck to classic reefs. However, there's almost no dive industry - or even tourism industry - to speak of on the island. The only resort with a dive operation I can find is the ultra-luxe Amanwana resort on Moyo Island.
The island's dive sites - especially those around Moyo and Satonda Islands - are mostly only visited by liveaboard dive boats traveling between Bali and Komodo. Some liveaboards depart from Bima, and these will usually do the sites around Sangean Volcano as well as the Unusual Suspects site near Bima.