What does the part called a "face seal" do?

[GailB]

The little rubber O ring that sits on top of the water pump impeller housing does what exactly? Obviously it is important, otherwise Mercury wouldn't have put it there, but what happens if it is not in place?

[Ski]

It's there to prevent exhaust from entering the impeller housing

[GailB]

I saw the lower unit less than a minute before the tech bolted it back on and the O ring wasn't on there at that time. He was a little distracted talking to someone I think was his wife. Since them I have what sounds like a dry bearing in the motor when it is running, especially at higher RPMs. It is not constant, but I hear the sound more often than not hearing it. Also, this impeller seems to pulse rapidly rather than have a smooth water flow out the telltale indicator. Does not having that in place sound like it could cause the problems I listed?