I just found the cruisemates boards but I posed the same questions you are asking on another board. Here's the address.
Also, when are you going? We'll be on the Grand Jan. 7. We could hook up and do St. John in style. The Watermelon Cay site mentioned in the thread above sounds great. For now, we've booked the Champagne Catamaran tour. We may change our mind when we get on board.
I think the biggest issue is when do you get off the boat. The ferry runs from Charlotte Amalie at 9:00 and is 45 mins to Cruz Bay. We arrive in port at 7 so we might be able to make the 8 or 9:00 ferry from Red Hook. You'll get a lot of extra time if you get there for the 8:00 and about 30 minutes if you are on the 9:00 from Red Hook.
Here's the ferry schedule,