Grounding boat’s DC system with outboard

[2gr8escape]

I’m installing the electrical system from scratch on an older csk catamaran that has a single retractable outboard.

House bank is Li, starter is lead acid. They are independent (no combiner) except for a dc-dc charger (Renogy), that has appropriate chemistry settings. Both batteries are led to a common negative bus as will be the AC system.

As the starting battery is connected to the negative bus it is “grounded” to sea water when the engine is down. Bu, at anchor and while sailing engine is up out of the water, do I need a separate grounding wire? This is tricky as seacocks are marelon.

Thanks in advance for your help!