What can cause low water pressure ? I have only 10psi when I am at 4500rpm running down the lake.

Week impeller, water pump housing needs replaced too, poppet valve weak or valve leaking. Best advice I can give you is to check the gauge first, then replace complete water pump, complete poppet assembly and both thermostats.
.

So what your saying is that even though the motor has a strong peeing flow at the Idle but low water pressure while on plane trimmed and running down the lake that it's the impeller.

Costa,, I AM NOT A OUTBOARD MECHANIC but I have a 2000 200hp Merc. I have had the same symptoms for the 10-12 years that I have owned the rig. Impeller normally get changed out at 2 years intervals. Even with new impeller gauge pressure is the same about 10 lbs at 3000 to 3500 rpm. I believe the gauge is INCORRECT. Like you said good peeing at idle with only about 1-2 lbs showing. Long story short I have quit worrying about it. Daniel

has it always been this way?.. did it decrease (if that applies) all of a sudden or over time?.. has tha buzzer ever went off?..........

The water pressure gauge is a tube that runs from your engine to your dash. Tubes can kinked, blocked, piece of sand can get wedged.

Before you start on anything else, take your cowling off, and inspect that tube from the motor to the gauge. I'll wager you have a leak or blockage.

I just bought the boat so I can't tell you what it did in the past. Talked to the prior owner and was told the impelled was changed about three years ago so I will be changing it soon. The motor has a smart gauge on it and the temp reading is fine so maybe it is will be alright after I change the impeller. It could be the way Daniel said and I just quit worrying about it as long as it's peeing good and the temp isn't too high.

water pump kits are cheap compared to losing a powerhead an 3 years is too long without one.