Huh, I've had my iphone apart twice, and even looking at the charging/docking port how the fuck can you tell its had water in it? If you take the screen off, there are two of those water dammage stickers on the motherboard but other than that I dont know how they could tell. If its still under warranty you need to bitch a little more, they will cave in, get a little unruly in the store and they will give you what you want just to hush up.
If its not under warranty, hopefully you've sync'd it with itunes recently, then you can pick up another motherboard for cheap and just slap it in there.