Welcome aboard, Tiffany!
Unfortunately, St. Maarten and San Juan are two ports in which Disney Cruise Line does not offer a dolphin swim Port Adventure. I don't believe there are any independent dolphin swims in San Juan at all, actually. In Aguilla, just off St. Maarten, there is a beautiful spot called Dolphin Discovery where families can swim and interact with dolphins. If your daughters have their heart set on swimming with dolphins on this trip, you might look into that on your own. You'll need to arrange your own travel, including ferry travel from St. Maarten over to Aguilla island. However, I normally don't recommend booking adventures that are not through Disney.
When guests book Port Adventures with Disney, they can relax knowing that Disney has taken care of all of the details. Plus, the ship always knows where you are, so if your adventure is running late, the ship isn't going to leave without you. Heading out on your own, you need to be very aware of time and be sure you're back before the last all aboard call. That makes me nervous, and I know guests out on their own have been left behind in the past when they aren't back in time. So, I always stick with Disney's adventures.
You might take a look at the exciting
Port Adventures that are offered through Disney in those two ports. A really unique experience in St. Maarten that I'd recommend is the
Sea Trek. It's such a funny feeling donning the helmet and sinking down to the bottom of the ocean where you'll breathe freely in your helmet and walk around the floor of the ocean while taking in all the beautiful scenery.
Have fun planning, Tiffany! Stop back with any other questions!