Here is a link to manuals for most Mainship models including the P30: https://drive.google.com/folderview?...kE&usp=sharing
The Yahoo Mainship Group is a great source of info even though I hate the Yahoo web site. Go to groups.yahoo.com, search for Mainship and sign up.
The Yanmar engine is one of the best they made but like all marine engines needs some maintenance to keep it running right. Even with only 350 hours of use, it has been sitting in sea water for more than ten years and that will take its toll.
Go through, check and clean and if necessary replace everything seawater related: raw water impeller, all heat exchangers in the raw water circuit, zincs and the exhaust elbow. You should particularly pay attention to the air cooler which if it fails internally can let your engine ingest sea water.
As cardude noted above, the Pilots have a marginally designed exhaust system that can let sea water back up into your turbo. You can check to see if water has been getting back in by removing the insulation blanket on the exhaust and unscrewing the V-Clamp bolts on the joint at the exhaust turbo exist and at the mixer joint. Take off the elbow and look inside the turbo throat. It should have no rust or corrosion and only a light coating of soot.