I'm in the process of connecting a water pressure gauge to a 1998 Mercury Offshore 150Hp, in the manual it says there is a gray hose coming out by the front of the lower cowling, I don't see it or anything like it. Is there a water port plug that I need to connect to? The gauge came with a brass threaded connector with a barb on one end for the hose. If this doesn't work for my model should I T into the hose coming off the top of the block that connects to the poppet valve? Just looking for the most accurate way to read the water pressure.

I don't think the 98 had the tubing installed yet.
T off of the hose coming from the top of the block, you'll be fine.