Sweetie you can't take the bus from Dar to Zanzibar unless they have paddles..
Zanzibar is a fabulous place. lived there with my parents. the best way is to go to Dar and take the Hydro-plane or the small commuter plane they have over to the island or if you are really adventurous they do have fishing dhows that will bring you over but it will take several hours and sometimes in certain areas the ocean can be very unpredictable.
The whole island has been built up with a lot of expensive hotels that cater to the tourists who like to have a luxurious vacatiion. there are many other places that are less but very nice some in old Stone Town..
Payment is TZ shillings and once you arrive on Zanzibar be prepared to have your bags thoroughly checked . (sometimes it doesn't hurt to have a 10US if you catch my drift)
The beaches are incredible, you can take a boat over to Prison Island which is lovely there are tortosies wandering around..it's peaceful. the big thing was to go to the harbour at night and drink sugar cane juice, fried octopus, banana chips..
Lots to see, the Sultans old palaces, the slave caves, listen why don't I just go with you and show you around. what the heck it's no bother really!!
I'll find more info and post it tonight...