Old bike inner tubes from the dumpster behind the bike shop, make great resistance training aids. Much safer than weights, with the same effect. Imagination and innovation will build any muscle with them.
Tubes and bands (bands have come a long way in recent years) are a staple of strength athletes today, especially for muscle conditioning where you need anaerobic stamina (Holding something under load for a long time) as well as general strength training.
You can mimic real-life strength applications with bands a lot easier than you can with free weights. There is no substitute for free weights when it comes to developing max power and size, but that isn't the OPs intended goal, so it's a moot point. You certainly aren't going to haul a Gold's Gym around in your boat.
There are some fantastic resistance bands available today. I used bike tubes for years and they still work as well as they always did, but there are some terrific options out there for a little bit of money.
Here's a link for anyone who is interested in bands: (no affiliation: picked this one because it is easy to navigate and because they offer a broad spectrum of bands)
Exercise Bands - Rubberbanditz