Plastic barrels won't get chewed up by muskrats like foam does.

They will probably float too high, making the centre of gravity of the
docks too high, and therefore the docks will be tippy. But you can
partially fill the barrels with water to lower the centre of gravity.
You have the advantage of putting letting just as much water into the
barrels as you need to lower the docks.

With water in the barrels the docks won't blow about in the wind.