Not that you asked for advice, but unless you have a pretty light bike, I would leave the moto on the main land. I was on an R-100GS and getting it on and off the ferry was really stressful and expensive. It's one price to get it on the ferry- but if you don't like it you can walk away. Getting it off the ferry is another thing. You have to do it and you can't do it alone so they rip you off like mad. I went to the boat Captain with this and he was on their side. You could rent a bike in Zanzibar for less and you wouldn't have the bike damage that I got from these guys hoisting it up and down twenty feet.
It's safe to leave it at the Bagamoyo beach resort (assuming John Doe is still there) and either go to Zanzibar from Bagamoyo or get a bus back to Dar and go from there.
Just a suggestion.