Ferry from Dar es Salaam to Zanzibar ?

I've been searching for flights from London to Zanzibar in October, but it seems like the only airlines that fly there are Air Ethiopia & Kenya Airlines, both of which have terrible online reviews & Qatar Air is very expensive. So I was thinking of flying to Dar & getting a ferry; is this a feasible option? If so which ferry companies would you guys recommend? I will have quite a bit of luggage as I'm probably bringing dive gear, so is there a luggage limit on the ferries? Thanks.