DAMAGED EXHAUST VALVE
This problem appears on both relatively new and older engines. Any situation that allows warm seawater or seawater vapor to reach an open exhaust valve that remains warm after the engine is shut down is a likely source of significant corrosion within minutes.
While the manifold cools after the engine is stopped, the seal created by the water present in the water lock can create a vacuum thus sucking in water that can settle on the exhaust valves.
This phenomenon can be seen before any disassembly of the engine, by measuring the compression ratio in each cylinder. White exhaust gases also show how too little compression can lead to incomplete fuel combustion.
This problem most often occurs on the cylinder closest to the exhaust. Many motors are installed inclined. In this case it is therefore the cylinder most at the rear that is damaged first in case of water inlet on the exhaust valves.
Intake valves can also be prone to corrosion, but since their temperature is much lower, the damage is much less even if it also causes a loss of compression.
Any decrease in valve tightness generates a loss of compression and therefore power.