I wonder how hard it would be to shrink wrap/vacuum pack/otherwise seal the foam. The issue with the different polystyrene foams isn't closed cell vs open cell, they are both closed cell but expanded polystyrene (beads) has many crevices between cells that are not closed. This provides more surface area for water intrusion. The tests that I've seen show that while it absorbs water much quicker than extruded polystyrene, it also dries out faster. Extruded polystyrene is also susceptible to water absorbsion if submerged. Polyisocyanate (urethane) foam is the type poured in. It is open cell on the inside but closed cell if you consider the whole mass a cell. It's my understanding that it is less susceptible to water intrusion than polystyrene, but if that outer skin gets damaged, then it becomes a sponge. I have no idea on the permeability of polyethylene (pool noodles I think), but would imagine it to be better than the rest. All that to say, it's a trade off, but would be nice to seal the options better.