I would use a low loft poly, as it dries so fast ... because poly does not really absorb any moisture. The oil based fiber never really gets truley wet. Even though you think it may/will not get wet , it will be good to not have issues if it does get wet either on the beach or in the "gear" that it may get next to in travel to and from.
I have a roll of low loft poly that is smaller than its comparable size cotton batting.