You're probably right. I've not been there myself - T&C that is, but having done StJ over the Christmas holidays, we thought the water and the breezes were COLD. And truthfully, since the family just did T&C this year and didn't complain about water temps, as well as my boss and his wife who just returned from two weeks there, I drew the conclusion it was warmer there.
That being the case, if I were the OP, and depending on where they're coming from, St. Maarten/St. Martin might be a great choice for them. It has all they're asking for and it's definitely warmer water temps than StJ, no?
"Always keep your words soft and sweet, just in case you have to eat