I re-read your note about restricting the PRV hoses with vise grips and the motor would cool itself. I am assuming this is during the cruising speed, and not the idle speed.
The PRV is intended to bleed off excess pressure when the engine speed goes up. It also is intended to close down flow to the risers and divert all the cool water into the motor first, before being ejected out the tail pipe. If you restrict flow to the PRV system, then you are forcing more water and pressure into the motor at higher speed. Personally, I don't think the additional pressure would hurt, but I'm beginning to wonder about hose size and the conditions of your conversion to Raw Water Cooling (or as CC says, Standard Cooling).
In your conversion, do you have a
" T " or an
" L " right off the sea water pump?
Regards,
Paul