We took our daughter on a Caribbean cruise at 3.5. She remembers being on the boat now at 7, but doesn't remember exactly what else happened there vs other trips. For instance: she remembers the "Disney boat" and remembers the dolphin encounter in the Bahamas, but forgot they were the same trip. She does remember the single day at MK we spent before driving to PC for the cruise and that they were the same trip.
The biggest issue we encountered was her bedtime. She was asleep by 7:30 most nights, so we had the first dinner seating and never went to any of the shows on that cruise. One of us would go out in the evening sometimes, but often we were exhausted and ready to recharge for the next day. Had it been just us or us with relatives, we also might have considered the midnight premiere of Big Hero 6.
She liked the kid's club when we would go with her during the open houses, but only did kid's club alone a couple of times. She was used to the kid's club environment from her preschool experience, but didn't get full enjoyment out of it. She wasn't thrilled the day of our Palo brunch, but was perfectly happy to go back alone another day. She definitely wasn't one of the kids insisting on kid's club and refusing to leave. She did love the pool. She also loved the characters and ate most of the food, although she's pickier now than she was then.
Now she's excited for her 2nd trip on the Disney boat next month when we go to Alaska.