My neighbor threw out a 6' diameter hottub cover, presumably because the
fabric covering had a small cut in it. It is entirely styrofoam.

I was thinking of using it either as a swimplatform at the lake, or as
floatation for a wood swimplatform. The manufacturer recommends against it;
although the covering is nominally waterproof (well, after I patch the cut)
the styrofoam will eventually absorb water and become useless.

Is that true? Styrofoam is used for boyancy in boats and it doesn't absorb
water, but maybe they waterproof it somehow. (wouldn't you think a hottub
cover would be waterproof?)