Are you speaking of the shift connector access plate? If so, nah no problem, even if it leaks. If you mean something else, please explain. And what motor?
It is number 13 in the linked diagram. Now, for the real question...why does it not matter? If water is ok to get in, why gasket anyhow? Oh, 18 hp fastwin.
The cover and gasket are designed to keep down exhaust leakage and noise.-------It does not do much other than that.-----And there certainly is no need to worry about water getting in.----That area is full off water when motor is stopped / boat is not moving
So really it sounds like I do not even need to replace it. When I first got it I noticed a string of what looked like rubber coming out from around it. Figured it was dried sealant material that had snuck out around the edges so did not want to take any chances. Explained this to the people I bought the part from. They agreed and said to replace it...nice! Oh well, guess I have one now if i need it in the future.