My suggestion is based on former car trouble. I replaced the timing chain and would subsequently lose oil pressure over 50mph. My conclusion was that I had dropped a shred of gasket into the oil pan and it was being sucked onto the oil pickup screen. Lowering speed would clear the problem. It's been years, but I think I cleared it with an oil change. Of course the drain hole on the V8 was larger than that on your boat engine. I knew for sure that it was a real drop because the hydraulic valves would knock when the pressure dropped.
I don't think the po's analysis is right. You're motor would have to lose a lot of oil through a mechanical fitting to cause a pressure drop.