what do I do if timing belt is off by one tooth

maybe it is the tensioner, unless maybe someone installed it off a tooth

Replace the belt and all timing components along with the water pump.

What makes you ask? How do you know it's off A tooth? 2.3--3.0?? CL right?

Take it back to the mechanic who replaced the belt, if it was just previously done. Who told you it was off one tooth? If you havent replaced the timing belt recently, you are at the mercy of the mechanic who told you it was off one tooth. Get a second oppionion from other mechanic you know is honest and reputable.