Even before you get to talking with Yanmar about the isolated ground, talk to Amel about your C-Drive. It has been mentioned several times here that the older C-Drives are not rated for greater than 100 HP. I have never talked to them first hand about this, but the information comes from reliable sources!
Now about grounding...A full answer to that question is complicated and somewhat nuanced. Let me try doing a short form...
The Amel has three electrical systems onboard (actually a few more depending on how you count, but this works for now).
The 24V DC system, the 220V AC system, and the bonding system. In the Amel concept these systems are designed to be completely independent, and have no cross connections. This minimizes the chance of problems with stray current corrosion, and with managing the problems that come about when plugged into shore power. Amel did violate this total isolation in a few places, but that's another story...
The engine block needs to be part of the bonding system for both lightening strike mitigation and corrosion control, especially of the C-drive components.
Most engines manufacturers also make the engine block part of the negative return circuit to the starting battery. If this is true, then you have connected the bonding circuit to the battery negative through the engine block defeating the isolated bonding circuit. This makes leakage currents (and all the very serious problems they can cause) both more likely, and much harder to detect.
I know when you are repowering it is a pain to put an isolated ground as a "must have," and many people will try to talk you out of it, but stick to it. It is important.
Engines can have isolated grounds in two ways. Have starting motors that are isolated from the block and have their own negative connection, or have a relay that opens the block ground when it is not needed. In either case, the alternator needs to be the isolated return type, and all the instrument senders need to be two-wire isolated ground.
All of the above information applies to the generator as well.
Fort Lauderdale, FL, USA