It is true they won't let her in the pools on board until she is potty trained, but she can still bop around in the children's section. There are some great beaches in St. Thomas and St. Maarten. The easiest to get to in St. Thomas will be Coki and Magen's Bay. The glass bottom boats are about 40' long, they are like a normal boat. The sides are about 3' high. She could stand up on the benches and lean over and fall out, but if you're holding on to her, she'll be ok. There is also an Atlantis submarine in St. Thomas, there is no way anyone can fall out of there. There is a tram ride that takes you to the top of the mountain in St. Thomas to look out over the area that might be fun, but she'd probably have more fun at the beach. There is plenty of shopping in both St. Thomas and St. Maarten, but the water and beaches are so beautiful. There's also Blackbeard's Castle, it's a tourist trap, but she might have fun there. In St. Maarten, the most famous beach is Orient Beach, it's on the other side of the island from where you dock. That way you can see some of the island. Orient is a clothing optional beach, and sometimes they stroll up and down, but at 16 months I doubt she'll even notice. Maho beach is where the planes almost land on the beach, it's pretty neat, but may be too noisey for your daughter. There's a beach near where you dock as well.
It's true that it won't be as relaxing for you and your hubby with a 16 month old, but y'all will still have a good time. It will all be a new experience for her. It will be hard on you though, especially the flight. If you have enough to keep her entertained on the flight, or can keep her up so she'll sleep, it might be a little easier. They have a cartoon channel on the tv in your room and there are movies too. On our Christmas cruise on the Dawn, they gave us a stocking with some candy in it. She'll think Santa found her even in the middle of the ocean. There's plenty for her to explore on the ship. I'm sure you'll receive other replies as well, best wishes!