Even if you do not make it to Mnemba, the lagoon directly off Matemwe rewards an afternoon's snorkel. At high tide the water inside the reef is two or three metres deep over patches of seagrass and small coral heads — easy snorkelling with sergeant majors, parrotfish, butterfly fish and the occasional small octopus or sea cucumber.
For a more dramatic experience, ask a local boatman for a short ride out to the reef break. Outside the reef the bottom drops sharply and the visibility opens up, with larger reef fish, bigger coral structures, and the occasional turtle. Conditions need to be right — a calm morning, near high tide — but the trip is short and cheap if arranged direct with a village boat (rather than through the dive shops).