I stayed in kendwa, nice beach, and there's always a beach even at high tide which is an issue on other parts of the island. It gets less crowded the further north you go. Regarding diving: Mnemba. Don't bother with anything else, Mnemba was so much better than the other dives we did. If I could go back I would just dive 4 days at Mnemba. Visibility was as good as it gets, on one dive saw turtles, octopus, two sharks, and dolphins! The local kendwa dives had very poor visibility and very little sea life.
It's not so simple from kendwa because boats will only make the trek if there's a certain number if divers, but if you don't care about being in a speed boat I think it's pretty easy to find a wooden boats but it's a long day.
So all that said, if the focus of your trip is diving, stay somewhere on the north east where you can easily dive Mnemba everday. But frankly if your main purpose is diving you should go to Pemba or Mafia islands. If staying in Kendwa the north end of the beach is much quieter, there are some hostels further south that are nice to visit but we enjoyed staying at kendwa breezes. food was not great anywhere, we also visited nungwi which has lots of restaurants on stilts over the water, very pretty. and of course spend a couple of days in stone town wandering the streets, check out the nightly fish market for some food on a stick.
