All the descriptions make sense, but now I'm confused. Like josrulz, my vent is in the anchor well which definitely gets wet. While quite a bit higher than the tanks and faucets, I now wonder why I don't receive any contamination. Perhaps the vent is a one-way valve. That's the only explanation that makes sense to me.
Hi Sabreman, if your set up is like mine (good chance it is), the vent is like a holding tank vent high on the bulkhead of the anchor locker. Although the locker gets wet inside, the vent is aimed down, so to get sea water in it, the anchor locker would have to literally fill up.
I don't know if I'm going to re-run it, but if I do, I like the sink location idea.