We did the NYC to Castaway and PC last year the week before Thanksgiving. The weather was cold in NYC but hot in Orlando. The water at Castaway was cooler than I like it, but I HATE cold water and I still got in and stayed in for a while because you really did get used to it after a minute or two. We don't ever really do the pools on board but all the kids were definitely in the pools all week. The water felt very smooth to me but I'm not usually effected by rough seas so I don't notice as much as others.
The WDW was so much fun. Like other posters have mentioned: you pre-select your first park of the day and there are buses waiting at PC to take you straight to that park. You'll receive a paper ticket in your room like you would for any other excursion that has your Park of choice and departure time for your bus on it. People are supposed to exit the ship in order based off that departure time but EVERYONE tried to line up super early so the line to get off was very very long. There was a hold up in the ship clearing customs so that made the wait even worse, but once we line started moving it went pretty quickly and we were on a bus and on our way to Animal Kingdom. We got there maybe at 9:15am (park opened at 9am).
Your Key to the World Card will be loaded with a 1-day Park Hopper ticket valid for that date only. It will also be loaded with 3 Anytime/Anywhere FastPasses. We could use them anywhere last year including Flights of Passage but I seriously doubt they'll be available for the Galaxy's Edge attraction this year. We used our 3 anytime FP+ on Slinky Dog Dash, Toy Story Midway Mania, and Seven Dwarfs Mine Train.
g-ppetto is right in that last year there was a Ticket # on the back of our Key to the World Card that we were able to link to our MyDisneyExperience account using the app before setting sail. Once it was linked we were able to make 3 additional FastPasses. We used them in the Magic Kingdom in the evening. So in total we had 6 FastPasses which really allowed us to do a lot. This worked for us with no issues but we knew others on the cruise who for whatever reason couldn't get the ticket to link to their MDE. I think they went to guest services but can't remember if it was resolved or not. You can use your KTTW card to tap to get into the parks or FastPass+ entrances or if the ticket is linked to your account you can use a Magic Band.
After the fireworks we walked to the buses outside the Magic Kingdom and there was a spot for
DCL. There were DCL Cast Members there to scan your KTTW card and give you a bottle of water. A little over an hour later we were back at PC. Got back on the ship around midnight. We were completely exhausted but had so much fun.
Let me know if you have any additional questions. We loved the cruise so much we're doing the same exact itinerary this year over Halloween.