I can see why you think it could be normal (I consider Normal the second most likely option), but like I said, there's too much mystery surrounding it for it to be Normal type, imo.
Water doesn't just "wash away" poison, though. The poison is inside the bodies of poison types, you can't wash that away, and water certainly doesn't affect it (aside from watering it down). Poison, on the other hand, gets into water and makes it undrinkable. I can see the poison getting into water types and totally screwing them over.
And while it doesn't make nearly as much sense, I wish Poison was resistant to water as well. There's only three things that resist water: water itself, grass, and dragon. Two of the three are disposed of by Ice attacks, and the other one arguably makes water even more overpowered, considering that it's one of the few things reliable enough to take on its own type.
Poison's not weak to ice, and unlike Water types, it has STAB that can actually hurt Water types a lot (assuming it was super effective as I mentioned before). Poison type Pokemon and poison type attacks would suddenly skyrocket in usage, considering how prominent Water-types are.
And as another added bonus, Poison types don't care about getting Toxic'd by the fairly common bulky waters; they're immune!
Last edited by Neosquid; 02-15-2013 at 04:24 PM.