Some years ago I did a battery upgrade project and as part of that I replaced some rotted subfloor up front below the window with a marine composite called Seaboard. Also sold under the name Starboard.
Although I sealed some leaks up front as part of that project, knowing that area was prone to leaks I was thinking ahead. I just used a strip about 1' wide and 4' long. The stuff is pretty heavy but for a small piece like that I don't think the extra weight would be a problem. I wouldn't use it for the entire subfloor though, just because of the added weight.
See my blog under 'battery upgrade' for a few photos of the subfloor work.
When life gives you lemons, don't make lemonade. Go get the oranges that you really want.