Swimming in turquoise waters... the best waters I have swum in... lovely in colour, not too cold and not too warm... just perfect. Many happy hours were spent this way. Sidenote: only in and arund Nungwi it's possible to swim most of the day, since this stretch of coastline is very little affected by tides (compared with the rest of the island).
Equipment: Swimsuit, please. Topless or nude bathing are not allowed for religious purposes.
For diving or snorkeling, the Spanish Dancer Dive Centre is the place to go.
Single dives start of 40 euros, and snorkeling trips and 20 euros. Snorkeling at Mnemba (which I did not do) is considered a must, and costs 30 euros.
Don't worry about not finding The people of the Dive Centre, they are on the beach and they will find you.
Equipment: Everthing (lunch included) is provided.