So I'll try to be helpful. If it was me, I would just bring a mask, reg and BC. The snorkel gear that Sunsail puts on the boats was actually nice gear so instead of hauling fins back and forth I would just use theirs. The masks they supply are nice two but I would prefer to use my own for diving. I can fit my travel BC, reg, SMB and mask in medium size carry on back pack. Obviously you will rent the tanks and weights.

As far as time for diving, I would certainly think you could make some time or coordinate diving and snorkeling with no problem. If he is a diehard diver, it would be hard to spend a week there and not dive some.