Since most boats stay in the same place virtually all the time, it falls to the bottom, under your keel. It builds up over time and you can harvest it, melt it down, and recast it in the proper shape. If you drape a big blue tarp under your boat, you can catch it and that makes collection much easier.
I'm working on a collection system for shoe and tire rubber. It's patent pending, but involves scraping the road or sidewalk (the pedestrian version) as you move.