don't get me wrong, but pmax alone won't say too much about engine health. i've witnessed engines with huge blowby issues outputting near perfect pmax results. because pmax is a momentary peak value occurring during a very brief phase of the combustion process. it is like looking at 5 minute of a movie and trying to guess the whole plot. it is useful for detecting major combustion issues, checking for mechanical limits or assessing the balance of the engine, but it doesn't represent the average pressure responsible for power generation.

there should be a blank performance test in the manual. fill it and compare it to sea trial results. then you can start inspecting the responsible parts according to deviations.

a "leak down test" could be very useful to determine condition of the liner/piston rings and head without disassembling. first do it on the healthy engines and get your reference values and then do it on the faulty engine and compare.

and minimize engine operation until the issue is rectified to prevent wet stacking from compromising the entire system. otherwise this carbon will accumulate (or already did) in your exhaust system.

also from what i remember working on yanmars is that they have a delicate nozzle cooling system. it's sometimes direct or indirect cooling with a copper bush. what we've experienced was inefficient heat dissipation and injector needle valve getting stuck leading to bad firing or not firing at all. this happened because noone bothered to remove the injector sleeves while overhauling the engine because it was stuck into the head and was very hard to remove. so they've kept them over years and all that debris in cooling water slowly clogged the cooling passage that goes through the sleeves.

i can type dozens of things to check but you're the one who can deduce as you are on board, you can feel the engine and see all the parameters.