Obviously, if anyone is allergic to anything, you need to contact them ahead of time...and you can also let them know what your drink preferences are, and most will try their best to have that available. Like my Pepsi for example. But most of the time, they will have a local "soda" in the cooler, because it's less expensive.
All you'll need is your snorkel gear, if you plan to snorkel, and towels. Be prepared to remove your shoes before you board....most are picky about that.
And the WILL stand and wait for a tip when you return.
We've chartered privately in St Thomas, twice. What company did you use? Usually, the food is really fantastic. The best was with the sailboat we chartered while we were on the Princess cruise.