I know this gets confusing because in the panel of your home the green/ground and white/neutral use the same buss bar. In a boat however connecting white/neutral and green/ground together can be very dangerous, yet I see it all the time. DIY's often assume white and green can go to the same buss bar just as it does in the home panel. The green is a safety ground and because shore power systems and cords are highly unreliable this is why the green/ground is tied to the boats DC system.

When I worked on large yachts at marinas I would often see the dock boy "fixing" the docks power post after Buffy and Skippy drove away from the dock in their new Sea Ray 380 with shore power still connected, thus ripping it out by the roots. Mistakes in dock power systems and shore power cord faults are quite wide spread hence the E-11 standard where green AC is tied to the ships ground. The "dock boy electrician" is also a good part of the reason we have the reverse polarity indicator standard..