Slips are great however if you don't have at least dockside air they can be hot and buggy. Leverick is the exception and generally has a decent breeze.
Your first day plan is not realistic. There is no real way to go from Roadtown to watermelon cay in one day. Even if you clear out of the BVI the night before assuming you can get the skippers passport it's a long haul to Cruz bay and back again to Watermelon Cay in one day including customs.
G