That's a difficult one. It is pretty annoying to read that these things seem to happen more often. A friend of mine had a similar problem with his dealer. The point is that once you've bought it and taken it away, it is difficult to prove that the water was there from the start. And that's what they can rely on unfortunately.
However, if the dealer is professional he should work that out with you, after all you are the customer and he lives on you so to speak. have you told him directly what you think about it? If not, do it!