The east coast of Zanzibar is known for stunning beaches and fickle tides. When the water's out, it's way out; when it comes back in, arrange a sailing trip on an outrigger canoe to skim along the shore. The lower tides reveal the hangouts of the ocean's monsters (only about as gruesome as particularly spiny starfish); but as the tides push you higher above the sand, the jewel tones of the water become even more spectacular. It's win-win.
Any hotel will be able to put a traveler in touch with a boat or captain, and prices are negotiable; expect to pay around $15 for an hour.