Not sure I would bother with the Dhow trip since they only go around the bay and from the Cornish you can see just about everything that you will see from a Dhow.
Souq Wakif is always a good place to go - explore the little allies as there as some interesting shops hidden away. Near the Souq on the seafront is the Museum of Islamic art, but it closes at 19.00 so you won't have much time there, but well worth a visit.