You will score huge points by offering to clean. Schlep tanks to the truck, take the garbage to the trash, keep yourself a great boat guest by keeping your "stuff in your corner" (quotes denote an expression that would turn out to be asterisks), bring snacks and water, offer beer when back at the dock. Rec-90 in Key West is almost 3 bucks a gallon, so it isn't cheap to fill the tank, but the guy is going anyway, so he's already invested in the fuel. Another way to score big is by casually mentioning that you would be willing to boat watch on subsequent trips. Back a million years ago when I learned to dive on an air force base in the middle of nowhere, we could take 4 divers, but had to maintain a boat watch on the boat. We rotated through the group of us so everyone sat out a dive a week. Until we found a young lady who wanted to tan topless while we were diving, but that's a whole 'nother story.
Anyway, point is, this shouldn't or needn't be a financial transaction. I would look at the situation and see if your boat owner has more money and wants to do fewer chores, or if the whole bunch is a budget operation. However you approach it, it's always appropriate to kick in some gas money. I would say that the equivalent you would pay for a spot on a DRIS boat is excessive.
