Welcome aboard, Francine! The summer will surely fly by now that you have a wonderful adventure on Disney Cruise Line ahead of you!
First time guests with Disney Cruise Line can begin booking extras like Port Adventures 75 days prior to sailing. Guests who have sailed with Disney before will be able to book sooner, so there is a chance that some of the adventures could book up, but you're in luck with the dolphin encounter for a few reasons.
You didn't mention which dolphin encounter you were hoping for, but there are two
shallow water dolphin interactions at Atlantis. One includes the Aquaventure water park, and the other includes time to explore Atlantis. If for some reason one is booked, you can most likely get the other one if you are more concerned with the dolphin part of the adventure than the water park or Atlantis.
In addition to that, Nassau also has dolphin interactions at Blue Lagoon. I will be taking my seven-year old daughter to the
Blue Lagoon Dolphin Swim this fall because starting at age 6 there, the guests get to have not only the interaction, but the swim and push with the dolphin in the deeper water (wearing life vests of course!). This adventure also includes time after the swim to explore and relax on the Blue Lagoon beach.
So, log on that first day that you are able, Francine, and hopefully you can get your first choice, but if not, you'll have a few other dolphin adventure options. Have fun as you continue to plan!