I believe it is a plastic, not sure how it was molded, but it was molded over a woven mat, probably kevlar. I know fiberglass when I see it as I have built things out of it. Its heavier than a fiberglass canoe that length also, 65 lbs. My kevlar and resin canoe, same length only weighed 45 lbs.
It doesn't look like it came into contact with anything caustic, just something heavy sitting on it for an extended period of time to where it may have stretched the plastic and now it has to assume the dimple position, one way or the other. Under load, like an heavy cooler it assumes its normal down position.
I guess I'll just have to get use to it, after all it was a freebie from a guy who was moving and didn't want to take it with him. I looked it up at Old Towne Canoes in Maine and it sold for $1500 new. So I guess I can't complain.
Just thought maybe someone had run into this problem and knew a good fix for it. I might just put a brace inside across the bottom and glue some outdoor carpeting down on the inside surface over it.
Man, when it gets wet, its gets real slick, fast, so it could use some non-slick surface to keep me from slipping and go swimming with the fishes! LOL!