Re: AC to DC grounding
OK, easy Bubba. I asked what the OP's intention is so we could determine whether he is installing an entire AC system on his boat or just installing the single battery charger as the only load. Yes, an AC system on a boat is required to include grounding/bonding of the engine and any other metallic parts that could become alive. Complete systems require accommodations for corrosion, etc as well as safety. The OP's installation will fall somewhere between a complete system that might require an isolation transformer/isolator and the installation of a single appliance, the charger.
Under ABYC's standard A-20 for chargers, the input and output of a charger is to be isolated by various means. One of the methods is by using a metallic shield that is required to be connected to the AC system ground conductor. Installing a conductor between the AC ground and the DC negative creates an obvious connection between the input and output. Perhaps this is not a concern if you have an isolated AC system on your boat, but may have consequences if the system is not isolated.
And yes, electrical fatalities occur at marinas regularly. Most marinas post "No Swimming" signs, though unfortunately they don't explain why. I certainly wouldn't allow anyone in my family to enter the water in an electrically serviced marina. The distinction for me is that if a cord connected charger is the only appliance, the likelihood of a damaged cord electrifying the engine on a fibreglass boat is remote. On the other hand, if you're going to install a thru-hull receptacle, a distribution system and permanent wiring to the charger, you should consider using an isolated system and grounding the AC system to the DC negative. - Grandad