I think you'd be better off building a small deck on top of 4 or more
"floats".
One commonly free item if you can find them is plastic 55gal drums.
They are super rugged, and seemingly aren't UV damaged. I see them
frequently in
floating docks that are quite old and in good condition.
bill

"toller" wrote in message
...
Would it make any sense to cutting the polyethylene off, letting it dry
out in the sun for a while and then maybe putting spar varnish on it?

I only need a swim platform about 3 months a year; it has 9 months to dry.