The Disney shore excursion information about the Dolphin Encounter is a little misleading - it does sound like you will be away from the ship from 9 til 5. In reality you have an option of returning much sooner.
We did this on our recent trip on the Wonder (3/17-20) and really enjoyed it. Be aware that the Disney arranged dolphin encounter is not a "swim" - for more information about the differences see their web site, dolphinencounters.com.
We left the ship around 9:15 AM, took a ferry over to Blue Lagoon Island, and after a short orientation session we were led to the area for our encounter. The whole thing probably took an hour. Afterwards you walk over to a nearby open air covered area where you get a lunch and then you are free to lounge on the beach, swim in the lagoon, rent floats and water trikes, etc.
We saw several groups doing the swim and it looked very cool. Don't think my 6 year old daughter was ready for that yet, but she definitely loved getting to pet, kiss, hug, and dance with a dolphin.
One mistake we made was staying too long - we wish we had gone back on the 2 PM ferry instead of staying at the beach and going back on the 4 PM ferry. As the afternoon progressed passengers from other ships began arriving and the crowd quickly became less family oriented. Sure the beach is nice, but you're going to be at
Castaway Cay the next day, so head back to the ship and enjoy some time by the pool!