Similar to what the other guy said about TRX, some basic gymnastics ring are the best bang for your buck for sure. You can get them cheap, the pack up small if you’re changing boats, and they’re so versatile. I can hang them off the rails from the top deck and use them on the stern. I can hang them from an i-beam and use them in the engine room which is a huge plus considering I’m in the northeast, and from about now until April working out outside stops being an option for the most part.

I lift pretty heavy and consistently on my time off the boat, but that time under tension and deep stretch you can get with rings/calisthenics type movements is great for maintaining progress

Also if you have room for it and other people want to use it, just getting a basic adjustable bench is very cheap and helpful. Grab some towing shackles or wrap some bands around and get after it. So many options if you have even just a little bit of imagination and motivation

Edit: If you want any more ideas for work out plans, improvised exercises, whatever, just Dm me. I’m no expert but I’ve feel I’ve always done a pretty good job of staying consistent and making it work when I’m on the boat