I have never been asked to show my YF certificate yet, but have heard that they have gone stricter about it recently, and it's a good shot to have anyway. Hep A is definetely recommended (not only for travel in developing countries, btw), Hep B is passed on via blood and other body fluids, so it's not as easy to contract, but since you never know when you may need a blood transfusion... Hep C? As far as I know there is no vaccine for it - are you sure, ScubaSamantha?
Malaria in Zanzibar has become less common, but it's still a risk area, so prophylaxis is recommended (Mefloquine (Lariam), Malarone and Doxycycline are all considered effective in East Africa).
US$ is still the preferred currency for tourist activities, and many hotels and dive shops quote their prices in $, but they will also accept Tsh, usually at the official exchange rate (which I think is more like 1100 to the $ these days). If you pay by CC you are often charged an extra 5%, and not everyone accepts them. At ScubaDo they do, but since their bank accounts are in Britain, the price will first be converted to GBP and then charged against your card in whatever your currency is, so you will lose in the exchange. Better pay cash, $ or Tsh.