We were on the Fantasy for a Spring break cruise last year. We didn't spend much time at the pools, there are so many things to do. If you are a planner, you can plan to avoid the biggest crowds. The elevators, though, are pretty bad and we usually just took the stairs. It's pretty easy to avoid crowds for meet and greets if you arrive early and don't mind waiting a bit longer to be first in line. Enjoy your cruise. I love it and can't wait to book again.
I know you have a lot of wonderful replies and hopefully your fears have been calmed a little! We first sailed on the Magic then the Wonder. So when we went on the 7 night Eastern on the Fantasy a few months ago I was a bit nervous about the crowds, particularly on Castaway Cay. I had nothing to be worried about though, the family beach didn't feel at all crowded. We have a summer cruise on the Dream coming up next and even though it is summer, I am not concerned about the crowds. Have a great cruise!
I didn't find the Fantasy to be any more crowded than the Wonder. On a summer cruise in the heat, the pools will be very crowded on both ships. That's one thing Disney did NOT do right. Pools are just too small!
We’ve done many cruises and sailed on all 4 Disney ships and my all-time favorite was the 7-night Eastern Caribbean on the Fantasy. The food, shows, spa, and ports are spectacular. St. Martin is gorgeous. My kids especially loved the digital scavenger hunt around the ship (it’s called Midship Detective Agency).
I've been on all 4 ships multiple times and I don't think I have ever felt crowded on a Disney ship. Well, maybe getting off at CC when we did the Eastern on the Fantasy as there was a huge line, but other than that I'd say 21 cruises have been crowd free. I have definitely felt more crowded walking around the parks at times.
My advice to avoid pool crowds is to go against the flow. Example is do the late seating for dinner. Pools/ slide really down after 4:30- 5. The only crowds I really saw were the sail away and picture lines on the final night. But I was always able to find places that were quiet. We are lucky as our family is a late bight type. At 10pm deck 4 was empty. So relaxing sipping my wine on a lounger.