[QUOTE=Dj Yoshi]Depends on the size of the pool and all. I'm sure 400PSI would **** up your internals pretty damn bad. Pool or not :p[/QUOTE]
Well, not that you can even ever experience 400 PSI of suction. In atmo the most you can get is a 14.7 PSI pressure differential and then the pump or whatever is just sucking vacuum. Even at the bottom of a 12 foot pool (assuming 12 feet of water = 1 atmosphere) the most suction you could ever experience is about 30 PSI. And it would take one hell of a pool pump to create vacuum.
Quick calculation: you'd have to be some THREE HUNDRED AND FIFTEEN FEET underwater to possibly get 400 PSI. Not a depth that many people attain (I do remember a friend telling me about some video where a scuba diver got his leg sucked into an underwater pipeline and basically got ripped apart because of the pressure difference. Since pipelines usually have more pressure than 0 PSI, and the diver likely was not 315 feet down, it would have been much less than the 400 PSI mentioned in the story.)
So yeah; moral of this post is that Yoshi is right. 400 PSI would mess you up. Except it wouldn't be so tame as to merely rip your intestines out. It would probably suck you right through the grille and turn you into ground beef. Fun stuff.
Now, 400 PSI of pressure going the other way is much more feasible. That would make you explode.
**DISCLAIMER** I still do not recommend getting your *** sucked by a pool filter. First of all, it's disgusting for the other people that use the pool, and secondly, well, yeah. It's just gross. So don't do it.