Snorkelling on the reefs around the islands and sandbanks is good quality and very popular. A sample trip from Zanzibar Town might include snorkelling on the reef near Bawe Island, a visit to Changuu (Prison Island) and a late afternoon barbecue on one of the sandbanks.
Most boats have snorkelling gear, although it's not always in the best condition. Check before you hire.
Around the islands, snorkellers usually reach the reefs from their boat (always better at low tide). On the sandbanks you can snorkel direct from the beach.