Wow I can't believe that ride on the dhow was 8 hours!
http://www.zanzibar.org/#guideAt this website it says:
"Boats to or from Dar-es-Salaam, Pemba, Tanga, and Mombasa
A recent law passed in Tanzania prohibits tourists from travelling in wooden crafts, which prevents tourists from taking dhows to or from the Tanzanian mainland. This law was passed after a German tourist pitched overboard from a dhow and drowned, ruining the fun for the rest of us who might want to spend eight to twelve hours on a wooden boat without shelter from the sun and without food, water, or plumbing facilities. It is a silly law and has silly exceptions such as sunset cruises around Zanzibar harbor such as is offered by Mtoni Marine.
Although dhows are no longer a possible means of transport for tourists in Tanzanian waters, there are many other boats to choose from. If you have your heart set on a dhow ride from mainland to island, you can legally board a dhow in Mombasa that will take you, most likely, to Pemba. The dhows have no services, toilets, or cafeterias and can be dangerous in high swells. Some people have had lovely dhow rides but if the winds die or are light, your trip can last well over eight hours."
I saw a lot of mention of at least 4 to as much as 9 hrs. This is the only site that had anything about restrictions of use.