bad service

Sent my car for a service a month back,ever since there's a herring in my car, so I sent it in to be checked, get a call back claiming there's water in the plugs, yet I did not wash the engine and the car is only a year old,I have a service, maintance plan and the car is still under warranty yet I have to pay the cost of repair out of my own pocket cause they say it doesn't cover the damage, so what the hell I'm I paying for if I did not cause the damage?