If you are like me your open bay areas on the boat are restricted to a passenger footspace and your rowers section You can make a nice enough floor out of the 2x2" cedar deck railings, some deck screws, and gorilla glue. You can even run a strap in the center from crossbar to crossbar so it can help support a lot of weight. Mom inlaw wont fall through and its less likely to retain sand and gravel as a footing surface
Hey, if you want me to take a dump in a box and mark it "guaranteed", I will. I got spare time.