They make "special needs" swim diapers, either disposable or reusable. They sell them at online stores that sell pool products.
http://pooltoy.com/swimdiapers1.html
There are both disposable and reusable alternatives.
Hope this helps!
WE ARE TAKING SEBASTIAN TO A WATERPARK IN DALLAS FOR HIS 5TH BIRTHDAY,BUT HE IS NOT POTTY TRAINED YET. WHEN HE WAS SMALLER WE USED TO USE LITTLE SWIMMERS SWIMMING PANTS THAT DONT ABSORBE WATER,BUT I CANT FIND ANY HIS SIZE,HE WEIGHS 50 LBS. ANYONE HAVE ANY SUGGESTIONS ? I WOULD REALLY APPRECIATE IT. THANKS!!!!!!