I really wouldn't try to go swimming with a Polyfil tail. At the least, saturating it with pool water might cause the stuffing to clump, and the shape of the tail would be destroyed if it does. Polyfil is polyester, not natural fiber, but I don't know whether it'd be resistant to mildew... and it would pick up chlorine either way.
I'm not into the pokeymans so I don't know what your tail is supposed to look like, but have you considered maybe cutting the interior from a pool noodle or a foam pool toy? Those are meant to withstand water and pool chemicals, and that stuff is lightweight and won't clump.
FF9 cosplay? Hit me up!
I now have a Tumblr
. Sometimes I use it.