There are many ferries and catamarans that can take you between Dar es Salaam and the Island.
Azam Marine,Sea Express, Sea Star, Seagull and Sepideh Megaspeed Liners are among the nicest available.
Azam runs at 14h30 and 16h15 on weekdays and Sea Express at 07h30 daily. Fares range $35 to $40 for non-residents, including a $5 Port Tax. First class is only $5 more expensive than economy and certainly worth the money. But not if you get seasick... as the first class is at the top and it's rocking more up there. :) Just by the way: Best seat for seasickness is in the lowest deck, in the middle, at the back.
Most ferries schedules allow you to do the return trip the same day. For instance leave Dar es Salaam at 07:30 and return from Zanzibar on the 16:30 ferry. That leaves plenty of time to explore the Stone Town, the museum and have a nice lunch.
The trip is beautiful and lasts about two hours approximately. However, if the weather is bad it can take much longer and the trip can be very unpleasant. If you suffer from seasickness you are advised to take some anti-seasickness pills prior to boarding. The on board personnel hands out free sickness bags at the start of the journey. Flying Horse makes the same journey for $20. However, it will take around 4.5 hours whereas the others take 1.5 hours.
You may be able to hire a private boat for cheaper, but the trip will take considerably longer and unless you know something about boats, you could be on a vessel that is not equipped for bad weather conditions or an emergency. Remember: you get what you pay for.
Be aware that the "porters" at the Dar ferry terminal will hassle you for money and expect tips for referring you to "the best boat." If you don't want their help, be forceful. The dock is a zoo -- a prime hangout for pickpockets.
Passport. Although Zanzibar is part of the Union it maintains its own immigration service and you need to have a valid passport to enter, even if you come from mainland Tanzania.
Zanzibar street scene - video in DivX Quality:
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