Got Trunk Water?

The other day I was putting sound deadener in my '05 GTP trunk and noticed that behind the passenger side wheelwell,under the trim and carpet, there was some sort of vent. Odd place for a vent right? Well, just below that vent was about a half gallon of water. There has never been any visible dampness in the trunk. If you own an 04-06 and maybe even earlier, you may want to look behind the trim and look in the wheelwell to see if you're sporting a trunk canteen like me. I'm just wondering if anyone knows what this vent is for, and if it would be bad to block off, since I believe this is where the water is from.