good question about how the valve gets stuck in full open

I reread your explanations...it seems as if "sitting" with the motor off heals the problem...not the oil filter. Can you confirm this? You said that you turned the engine off for 10 minutes and upon restarting, you had good pressure. Usually, time is heat on a motor.

Taking another direction on the problem, how long does it normally take (after starting) for the pressure to go to low? Does it always happen OR are there instances where the pressure stayed up for the entire duration of your trip?

You would think that, if the attitude of the boat made the presure drop (nose up, oil moves to back of pan) then the process of slowing down would change the situation. Also, if the oil was being pumped to the top end and some obstruction was keeping the oil from returning to the pan, it would happen consistently.