I can think of no reason to bond the neutral to the boat DC ground. You are actually creating a hazard that doesn't need to be there when the boat is underway. I would also drive a local ground rod at the dock to bleed off any objectionable currents in the EGC.
I didn't follow your response so I'd like to ask some questions. If the vessel in question has an AC electrical system that is supplied by an on board derived system it would need to have a grounding connection to the hull grounding plate. That same plate is the grounding point for the DC system, the lightning protection, and the radio / radar systems. The AC wiring of the on board system will be supplied by a shore line when the vessel is at the dock. Even if a multi pole transfer switch that switches all the current carrying conductors is installed the Equipment Grounding Conductor of the shore line will continue to carry some of the neutral return current of other vessels in the marina that have their neutral conductor bonded to the hull plate during shore line operation. If I keep the EGC of the on board AC wiring aloof from hull ground then there will be no fault clearing path if an AC conductor faults to the metal hardware of the vessel during shore line operation. Given these conditions how do I guard against a fault to the vessels metallic systems during shoreline operation? What would having a supplementary grounding electrode bonded to the EGC at the dock do?
The objectionable current is divided neutral return current that is getting into the water from neutrals that are improperly bonded to hull grounding plates during shore line operation. Even if the wiring on some boats is corrected to avoid this connection it is in the nature of the marina industry that vessels will come and go. It would seem that the marina operator does not have an effective way to prevent this stray current flow.
If the receptacle that supplies the shore line is equipped with Ground Fault Circuit Interrupter (GFCI) protection then any fault is going to trip the GFCI because the water is at least likely to carry the five milliamperes that are needed to trip out the circuit. If all the outlets to which a shore line can be connected in the marina are GFCI protected then vessels that have the neutral bonded to the hull plate would not be able to operate from those outlets because there would likely be enough stray current through the water to trip the shoreline GFCI. That would avoid having neutral current flowing through the water but it also might cut down the use of that marina by visiting boats.