I find the snorkel excursions enjoyable. After all this is the Caribbean. The fish are colorful, The water is bathtub warm. Bring a waterproof sealable swim bag for your cell phone, and take pictures.
Basic agenda from 4 different excursions (one on St Lucia).
Walk from the ship to a big pontoon boat (14' wide, 40'long). Motor along the harbor to some wreckage placed on the harbor floor as reef foundation. Put on an orally inflated plastic vest (think airline life vest without the inflator or light). Float on the surface looking at the junk 10 to 25 feet down on the bottom. Highlights may be fish, stingrays, turtles, or maybe even a sunken ship hull. Drink rum punch on the way back to the pier.
Please remember to put your "reef safe" sunscreen on before departing the ship. Please bring water and a towel from the ship. For the health conscious, consider bringing your own mask and snorkel, or you'll be kissing cousins with guests from the past excursions. That safety vest is not optional, and again, if you blow into the vest tube, again you're kissing cousins.
For the experienced free divers, that vest is not optional, so disassemble the tube and flood the vest to ensure all the air is out, and bring your own ballast if needed because it is not supplied. Otherwise you'll be bobbing around on the surface with the tourists gradually turning into a lobster.