I actually use a personal trainer on a regular basis. He charges $15/hr. I think the trainer on the ship charges $65. On our last cruise I never even went to check out the workout room. We just always used the stairs (I'd race the kids up and down!). I ended up gaining back about 6 pounds - which wasn't much considering how much we ate!
IMHO - I think you'd be better off to look for a trainer at home. Or spend your $65 on a months membership to a local club.
Besides - your on VACATION! Have a great time and don't worry about it!