Welcome aboard, Lisa! Isn't the Disney Dream a wonderful ship? I'm happy to hear that your family had such a great cruise on the Dream and such fun while exploring Atlantis' Aquaventure Water Park in Nassau.
The Disney Fantasy is currently scheduled for sailings to the Eastern Caribbean and the Western Caribbean. I don't know of any waterparks that are part of the Eastern Caribbean Port Adventures right now. And, honestly, nothing will compare with Atlantis' Aquaventure, as it is really a huge water park, but I do have one suggestion in Cozumel if your Fantasy cruise is on the Western itinerary.
Earlier this year, my family and I had a great time at
Playa Mia Beach Resort in Cozumel. There was something for everyone on this adventure. A beautiful beach with lots of place to relax for the adults, tons of watersport offerings like aquatic icebergs to climb in the ocean and floating trampolines. There is also a pretty cool water park area called Bucaneer's Bay for the little ones, but also hosting a fun Twin Twister waterslide for the more daring folks! Like I mentioned, this is not nearly the size of the water park at Atlantis, but it is a beautiful spot in the Western Caribbean and we really enjoyed a nice day at Playa Mia this past winter.
You might also check out the zipline adventures if your kids are the daring sort. You'll find a few different zipline adventures in the Western Caribbean, for different ages, depending on how old your kiddos are. We love to try different types of adventures in each port so that we can experience as much as possible, so even if you don't see a water park that interests your kids, I guarantee there is another type of Port Adventure that will thrill them instead, Lisa!
Happy planning!