I just got back from a VI cruise with Royal Carribean. They had two formal nights, the rest were casual. On formal nights I have seen men from one extreme to the other. Tuxedos all the way down to business casual but most people dress really nice for at least the first formal. I wore a long formal dress for first one and short cocktail dress for second. Hubby wore suit and tie for first one and simply a button dress shirt with a tie for second. Casual dinners I have gone in with jeans, sandals and a simple top and did not feel weird or out of place and the staff does not say anything to you. I wore sun dresses the rest of the casual dinners. Hubby wore jeans, sneakers and collared shirt like a polo or button down. No problems!
Carnival is even more easy going. I have even worn short sets to dinners and never had a problem. As long as you look neat, clean and tidy and you are not wearing your bathing suit, and your hubby's hairy armpits, back or chest isn't sticking out they are fine. Many ships have tuxedo rental right on them so if hubby doesn't want to pack a suit but wants to get dressed up he can rent a tux. Really if he just wears a nice button down collared shirt with a tie and a nice pair of khaki type pants he will be fine for formal night. On casual nights just a simple polo shirt and jeans are fine. It really is fun to get all glammed up for at least one of the dinners.
In St. Thomas we went to a place called Sapphire Bay for snorkling it was GREAT! St. Maarten we rented a Harley and rode all over but never made it to Orient Bay (the nude beach).