You can get them at any hotel, but it's worth shopping around a bit to either get a cheaper tour or a private one according to your preferences. The local Coral Gardens (2-3 hr boat trip from TSh60,000 per boat or TSh15,000 per person in a group) are relatively small, heavily damaged from fishing and tourism and the fish populations are very low, so it may pay off to take a full day trip round the northern tip of Zanzibar to Mnemba Island on the east coast (US$17-20). These trips are on the steady wooden dhows that are built in Nungwi with even the beams and benches hewn from trees. Although motorised, sails are hoisted to utilise the wind.
Offers various watersport activities; you can take a flight on a microlight boat, parasail, scuba dive, jetski, flyboard, ringo, banana boat or SUP.