See my write up on Zanzibar. I recommend that you go north and look at am offering on a fully inclusive basis. When you calculate the costs at the end of the day - the fully inclusive package workes out cheaper. Also have a look at the moon phase when planning a trip to Zanzibar as this will affect your diving.
You are welcome to contact me at Honey Coastline on honeycoast@mweb.co.za should you have any more questions.
I hope you have a wonderful trip.