cruise ships have gyms in them. and i would hit the gym a couple times in there, you will have downtime to go there while the ship is about to leave. really the first day there isn't shit to do anyway until the ship leaves the port so you will have free time here and there.
plus the mornings the ship is gonna be dead as well as everyone is lazy and sleeps in, so you can hit the gym in the mornings. you are bridging so don't let your conditioning go to hell. if anything i would get in a good workout and take advantage of the buffet food on board to load up in carbs and protein. the only thing i like about cruises is the unlimited buffet food!