I got an MPG drop after my intake install. I went from an average of 19.5 MPG (mostly city driving) down to 18.0 MPG after the install. I would chalk up about .5 MPG to the fact that I drove the car a little harder after the install so that I could enjoy the whine of my supercharger, so about a 1 MPG drop overall, but a nice boost in performance, and a good hit of pure fun.
As for the water, I have often wondered about putting some sort of "deflector" inside the fender that would prevent water from getting splashed up inside from a large puddle or something, but I haven't done it. When I do take my car to the car wash (for those times that I don't have the time to wash it by hand) I make sure that I don't get the "undercarriage wash"... No sense in having the car wash spray pressurized water in my intake area!
Other than that, you should be fine. Small puddles are no problem since the water doesn't splash up until the tires (behind the intake area) hit it.
