Oil Pressure 318 Chrysler

We've had our '68 SN out three times now. It has a 1968 318 Chrysler. I changed the oil and filter before we took it out the first time. Oil level is good on the dip stick. Oil pressure is fine when the boat is up and running, but when you bring her back down to idle speed the gauge maxes out on the high end. I took off again and the gauge went back to normal. This pattern repeated itself everytime. Any thoughts?

Put on a mechanical guage and check to be sure, oil pressure should be at least 10 lbs for every thousand RPM. Good oil should stay the same pressure after running hard or slow. I use 15-50 Mobil 1 and it nevers change for hours and hours.
Most straight weight oils wil drop and not hold   good pressure after hard running. However if your engine has excessive bearing wear, rods, mains or cam and or pump nothing will stay constant.