Because, if you have insulated gear and breathing aparatus that saves you from contact with water, you CAN actually still stay in there for longer times than normal.
But surfing? Yeah, right, if you wanna go out looking like Jacques Custoe going down to shake hands with the fishes, sure, be my guest.
Furthermore, of course there are going to be some bodies of water that are still mostly clean, but those are lakes and rivers in the woods/mountains, where close to nobody ever goes, aside from some natives . .