I have done the Dolphin Encounter at the Blue Lagoon twice. Several people we were on vacation with did the Dolphin Swim. We will be in Nassau in about 2 weeks and if we decide to do it again, we would choose Blue Lagoon. You begin with a really nice boat ride to the lagoon. I am not a big fan of Atlantis as we feel it is way overpriced for what you get. Check out the websiet for Blue Lagoon - Dolphinswims.com - lot of info there along with pictures and videos.
We are sailing on the Miracle out of NYC on August 28th. One of our ports will be Nassau. Have done 6 land vacations to Nassau and 2 cruise stops. This will be our 3rd cruise stop at that port. The only thing I can say that I don't like there is the Straw Market. Hot, crowded and very pushy vendors. You can do just as well looking for souvenirs in gift shops. Some items were actually cheaper in the stores.
Do you book the dolphin excursion through the cruise line or directly with Blue Lagoon? I noticed that it is cheaper with Blue Lagoon. Why so much more with cruise line? Is there a catch? Will I miss the boat coming back
Carnival used to offer an excursion to Blue Lagoon but they no longer offer it. They push for Atlantis and offer dolphin swims at Atlantis. When we did the Blue Lagoon, we were on a land-based vacation. It would depend upon how many hours you are in port, if you arrive on time and be sure to schedule it to allow yourselves enough time to make it back to the ship. If your cruise line offers it then you will definitely make it back to the ship. If you book Atlantis through your cruise line, then you will have no worry about making it back in time. It is what you feel most comfortable with. The web site for Blue Lagoon lists the various times of the day they offer this trip.
Good luck with whatever you decide to do. What ship are you on and where does it leave from?