The plywood sole of my 1968 Pearson Wanderer 30 was rotten in places and overall pretty well thrashed.
So a few months ago, I tore it out.
I used a reciprocating saw (a/k/a "Sawzall" or "saber saw") to take it out in chunks. It had been held down with a thick fiberglass and resin tab all the way around the perimeter, and bedded in a thick resin bed.
So after I tore out the floor, I was left with that thick tab all the way around the perimeter. I used a chisel to remove most of it:
Then I used an angle grinder with coarse sandpaper to grind down what was left: