First of all there is no hard and fast rule. Many different factors come into play in determining which side of the ship will face the dock. For St. Maarten and San Juan it could be either side. For Labadee it will most likely be the starboard side if you are the only ship docked that day. If two ships will arrive, then it could be either side. I have not been to St. Kitts.
Here is a picture of four ships docked in St. Maarten. Note that although there are two ships on each side of the dock, only one ship (front right) is docked with the starboard side facing the dock. The other three are docked with the port side facing the dock because one of the ships choose to back in ...
Front left is the Caribbean Princess. Back left is the Carnival Victory. Front right is the Carnival Glory. Back right is the Explorer of the Seas.
Two years later there were only two ships in port, and as you can see, they didn't agree on which way to face ...