No biggie, all this is info only. It's all good.

I put a hose with a straight nozzle in the hose off sea water pump to out drive to flush it. I noticed water was not going out as fast as it should have hence blockage. That hose can also collapse from suction if it's to soft.

OK.. I did some more "stuff".. Let you guys know after a little test run in a few days.

I've seen the inside of perfectly good appearing hoses delaminate , fluid will go inside the delamination and bulge inwards .Effectively blocking the hose ,causing a restriction , either suction or pressure .
The Hydraulic guys call it a Flapper.

Since I swapped intake lines from side to side, the hose is not an issue. (read above)

After another new impeller and housing, checked ALL the hoses, (clear) checked the oil cooler ports, (clear) fuel cooler ports, (clear) heat exchanger ports, (clear) restrictor at the headers, (clear) still have low water pressure. Rigged a temp. external water pick up, (by passing the outdrive) pressure is good but at 60 MPH. 4000 RPM it hits 40 PSI. Too high. I am totally stumped. Any other ideas?

40 psi is within specs for a 525. The manual says max is 43psi and I know of one person who regularly sees 60 psi.

Earlier post referenced that 40+- PSI is too high. If I don't have to worry about the 40PSI with direct water intake, (eliminating the outdrive intake, and allowing the water to be forced in) I'm ok. But.. here is the "but" if the direct water intake goes to 40 psi, that shows a restriction to me. There is NO restriction before the sensor. If there is some restriction after the oil cooler, wouldn't that show a "high pressure" ? instead of low?

This must be incredibly frustrating. I know you've tried a lot of things already, but there has to be something you're missing. If you go with the direct water pickup, you're treating the symptom. Do you have a sea strainer? Have you tried bypassing the strainer but still pulling from the drive? Maybe there is an air leak there. Does your strainer have a relief valve? Maybe that has failed and is allowing air in which is bleeding off suction. Have you tried swapping the water pressure sensors on the engines to see if the problem follows or stays with the engine ? Are you sure the core of the heat exchanger is unclogged?

Yes, yes, and yesss,, lol. But thanks. Sea strainer is bypassed for now. Already swapped port to starboard drive intake, (extended the hoses and drawing water from port to starboard, problem remains with starboard drive. Sensors been swapped. Getting 40+ PSI with the direct intake shows that the sensor works. Backflushing from the headers connecting hose (last point of restriction) water flows freely. I'm stumped . I just hope some one tells me what I'm not seeing yet, or didn't try. There got to be something somewhere. GGGGRRrrrr.