Are you talking about the Tortola Dolphin Encounter? You do know that while you leave from St. Thomas, the excursion is actually in Tortola. As such, all adults need a passport to do it.
After that... It isn't 'swim' with the dolphin. Yes, you are in the water with them (about knee/waist deep) but they won't have you hold onto their fins and be dragged through the water, pushed by the soles of your feet etc. That level of 'encounter' is definitely more expensive (but really cool. We loved it.) This one you learn more about the dolphins, you can pet them, get kissed by them, get your photo taken with them etc.