Knock sensor help

[olegk]

I have a 2003 hummer h2, with 50,000 miles on it. I was driving down the highway a last week, and my check engine light came on. I have been offroading and going through water the day before so i figured it had something to do with that.
i took the Hummer a GM certified repair shop and they said the light had to do with the Knock sensor. This will cost around $500 to replace. i declined replacing it and decided to do some research on it. the car runs fine, but i noticed when i floor it of a stop, the engine lags bigtime.
my question is: Is it nesessary to replace the knock sensor, and is there going to be any damage to my car if i dont? and also how hard would it be to do it my self? (i have decent mechanic skills), the technicians said the job requires that the intake manifold be taken off. Is it worth risking doing it yourself or is it better to play it safe and let the professionals do it?