I'm surprised more people are not interested in weight /resistance training, for non-livaboards. Not expecting that anyone should have a set of weights on board but most cardio programs don't do a lot for upper body strength. In any case I went looking for a piece of exercise equipment I had in high school.
Didn't think they were still available. Was expecting to find an old one on eBay. I think they would work pretty well on a boat. Low weight and don't take a lot of room and don't weigh much. Most of the exercises are a kind of combination of an isometric an standard weight training.