Most Unusual ............................
..................... but we could philosophise how this occurred this until hell freezes over, the question is HOW TO REPAIR IT ?
My suggestion would be to clean out as much of the disintegrated plywood, fiberglass, etc. as possible. This is going to involve many hours of hanging upside down in the locker. You may also be able to attack this from inside the V berth by removing any covers in this area and going at it.
Next, I would suggest precisely fitting and inserting a piece of 3//4" plywood or thicker (stack and glue). The piece should fit in like a cork such that it wedges in place as the bottom of the locker is like an inverted pyramid. Once properly shaped, it should be liberally glued in place with with epoxy glue.
Next is to seal the whole plug with a layer of several inches of pour in epoxy. The attached manuals will steer you in the right direction for products. Any brand of epoxy (not just System Three) will do as long as they publish good .PDF manuals. Most manufacturers of epoxy products do.
This should be an inexpensive but labour intensive job. Welcome to the wonderful world of boating :cussing:.