How much is to much water pressure? I have imco lowers with the inlet at the tip of the bullet. While running I can be over 30 psi. How much is too much?

30psi is alot. I'd back it down a bit, by installing a relief valve. Or, depending on your engine, adding steam relief lines to the back of the intake manifold water jackets.

the mercury pressure relief is set for 30psi.

if they are raw water cooled engines then that is alot. if they are closed cooling then that is right around where mercury wants them to prevent hot spots in the headers. they may have fixed the problems by now but the early HP525 headers had problems if you ran the pressure too low.

It is raw water cooled and pressure increases as my speed increases. Where is my water pressure measured? Where would I install the pressure relief valve?

I have one adjustable one from Eddie Marine and one expensive plastic Merc one. Merc one is always 30 lbs
the adjustable run I run at 15 lbs.

I would recomment Eddie Marine and since you are in fresh water, after the oil cooler before it enters engine.

And if this a relief valve where do I plumb the relief to?

Most plum them in the sea strainer and then blow off overboard. I would set at 20 psi for open cooled engine.