Jokes aside, I'd suggest St Lucia. However, I'd really suggest walking a block away from the cruise ship, catching a cab to a dive shop relatively not close to the ship, and explaining that you have to be back to the ship by (whatever time). Also, I'd suggest not necessarily planning an island just based off which one is prettiest, but which one will give you the best dive op. Maybe a dive shop is located right by the beach your family/friends plan on chilling at on that shop. Maybe that happens to be on the island where you have the longest amount of time on the island. That could mean 3 dives!
What is "their" purpose on going on that cruise? My family goes to sit on white beaches and swim in blue water. My friends go to get drunk on white beaches and look at blue water. Other friends go to look at the tropical, mountainous scenery. I go to walk over white beaches to go into blue water to look at fishies in it. What they're there for also changes what your most convenient dive spot is. I also have to say, everywhere you're talking about is GORGEOUS....you'll be happy at any of them. I say dive your most convenient location.