I have a 1995 Mercury 2.5L 150HP XR6 outboard, and I put a new water pump impeller in it this sunday.  I took it to the lake yesterday, and started it for the first time since fall and the new water pump impeller, well it would only spit water at idle, but it would spray water when i gave it gas in nuetral.  The thing is the water pressure gauge on the dash never showed anything, what makes that gauge work? and why would it not spray water at idle?

First thing I would do is make sure the P tube is not partially clogged. Dirt dobbers love ready made holes to build nest in.

The pressure guage is reading the water pressure on the primary side of the cooling system, if you don't have pressure, and have poor water flow out if the P tube, you dang sure don't want to run it until you find the problem. You can always get you a cheap 30 psi guage and connect it back at the motor by taking the hose off that feeds the dash guage and connecting your test guage there. Or, you can take the hose off at the motor feeding the dash guage and apply about 25 psi of air pressure to it and see if it reads the pressure you are applying.

If the hole and the hose from the motor to the P tube is clear, and no/low pressure on the test guage, then I would have to wonder about the impeller installation.

I had the same problem on a 200 merc after having it in the shop. So I took

the lower unit off myself  3 times, I would put it back together just like I

took it apart. Always the same problem, then I looked real close and notice the

gasket on the water pump housing. This gasket can be put on backwards.

The best way to describe this to you is to take it apart  and look at the gasket.

you will see a big hole and a small hole. Whan you put it back on, look in the little hole and make sure it is inline with the small hole. If not just flip the gasket over. What happens when it is installed wrong you will get some water pressure

but not enough. 8-)