Both my boys get bad rashes from something in ocean water. It seems coupled with a rubbing issue though with Nicholas it's skin-on-skin contact on his thighs and with Jonathon it's skin-on-suit contact near his knees. There's a large swath of red skin with tiny pinprick rash marks, unlike traditional "swimmer's itch" with distinct papules.
Whatever it is, it's frustrating. Our last day of playing at the beach Jonathon managed about 15 minutes in the water before declaring he hurt too much and would just play in the sand. Nicholas pushed through the pain but was really miserable every day as soon as he got wet with salt water.