Calisthenics would work. As would isometrics.

You might try a set of those rugger loops. They can apply considerable amounts of resistance. Often more than 100 pounds. Hook one of the stronger bands around a winch and pull or punch against it.

I wouldn't give up on weight training. A five gallon bucket of seawater is what, better than 45 pounds. You have an anchor that is pretty hefty. Tie weight to one end of a halyard and pull on the other. You can even tie a Swiss seat onto one end and haul yourself or your girlfriend up the mast.

More than one sailor has built long oars and work the body rowing with one or two. Good upper body workout.

Swimming is good exercise. Scrub the hull to take off the slime a couple times a week and your exercising.

Depending on the room available you might fit in a folding stationary bicycle. Make good use of it by rigging a small dynamo and charge a battery.

I have seen a guy who would anchor out and then pull the boat several hundred feet up the anchor line working like he was climbing the rope. When He got close to the anchor he would let the boat drift back and then do it again. Substitute stirrups on prussic knots and you could work the legs.

I also wouldn't give up on free weights if that is what you like. A sailboat is full of heavy objects, anchors and batteries to start with, that need to be kept low and fastened in place to be safe in heavy weather. Build yourself a sturdy rack to hold the weights and make provisions for the weights to be lashed down tightly. A simple engineering problem.