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For $20 - $25 (at least for locals, a bit more expensive for foreigners??) you can take a boat from Nungwi to Mnemba Island for some really nice snorkling. If you are lucky you can even pass by a pod of Dolfins on your way to Nemba. After Snorkling for approximatly an hour you are taken to another beach where you have your lunch.
This is a great half day activity. You leave your hotel at around 9:00am and are back by 3:00pm.