HP Failures are only caused by insufficient cooling
in the condenser. Maybe a bad pressure switch, but this is very rare, and would be very unlikely to give the symptoms you describe.
You say the flow is "good." "Good" is not a number. Get a number.
Collect it in a bucket and calculate how many gallons per hour are flowing and compare this the the manufactuers specification. What might be just fine when seawater temps are in the 70s might not work
when they are in the high 80s
If possible, check the temperature of the water
coming out of the AC unit. Hopefully you can find a specification for this. If this is above specification, you need more flow.
Lastly, a check for possible fouling of the interior
of the condenser: You can do this by hand, but a IR temp meter is more precise: Check the temperature of the line carrying refrigerant out of the condenser. It should be very close to the temperature of the incoming water.
If the refrigerant is still hot coming out of the condenser, there are only three possibilities:
1. water flow is not sufficient. Clean circuit or get bigger pump.
2. the condenser is fouled. Clean it.
3. the condenser is undersized for the incoming seawater temperature. You are stuck...
One more thing I saw once on an owner installed AC unit. The water connections were BACKWARDS. This provided sufficient cooling
at low seawater temperatures, but not when they climbed... Worth a quick check as an easy fix...