Compared to picking out china patterns, crafting the perfect playlist for the reception or figuring out where to seat Aunt Edna, the logistics of making the marriage legal in Zanzibar are fairly straightforward, but you will still have to jump through some hoops.
Most importantly, the only legally recognized weddings preformed in Zanzibar are civil ceremonies. All other wedding ceremonies - traditional, religious, secular, Star Wars - are not legally recognized. This means a Reverend Dave wedding, while beautiful and memorable and uniquely you, is not legally binding.
As a result, most couples opt to supplement their dream wedding on the beach with a short civil ceremony for local government records. These ceremonies, performed by the Regional Commissioner, can happen at the Regional Commissioner's office or if it can be arranged it can be possible to have the Commissioner come to the wedding venue.
Further details on the necessary paperwork, identification, required period of advanced notice and other requirements for a legally recognized wedding in Zanzibar can be found at the below site.
As mentioned, it is recommended that you or the resort where the wedding will take place contact the Government of Zanzibar to discuss the legal details of your upcoming nuptial.