Welcome aboard, Rachel! What a lucky friend to get to join your daughter and family on this magical Disney Cruise Line vacation! My daughter would love to bring a friend along with her, or join a friend's family on a Disney Cruise Line voyage. Those two are in for all sorts of fun both onboard and in the ports of call.
You didn't mention which ship you'll be on or which itinerary, so I can't give you suggestions in a specific port. But, most ports that offer a dolphin swim
Port Adventure have multiple locations offering similar adventures, so you might browse those in the same port and in all of your other ports to find the one that best fits your budget. I know it can be a bit pricey, especially because under 18, children need an adult to accompany them. However, for most of the dolphin swims, children ages 12-17 can do the swim on their own as long as their parent or other responsible adult accompanies them on the Dolphin Observer adventure at the same time which costs less than the swim, so that will be some savings if you're not set on swimming yourself.
While you could book a dolphin swim in a port on your own and save a bit of money, I'd still highly recommend booking through Disney Cruise Line, as your transportation is all included, and the ship always knows where you are. This is important so that if you are running late, you have less risk of missing the ship since Disney's Port Adventures keep in touch with the ship. Plus, letting Disney take care of all of the details really makes things seamless and easy.
My daughter and I had the most magical time swimming with the
dolphins at Blue Lagoon in Nassau, so I can highly recommend this amazing adventure! Have fun planning, Rachel!
Marcy