Can someone explain the purpose (in the drawing) of the AC GRD being connected between shore and inverter loads.
One example that might be helpful is the inverter itself contains both shore power (input) and inverter power (output) AC circuits. At times they may both be in use at the same time, e.g. charging while inverting, or otherwise present at the same time (even if not in use).
One can also come up with other scenarios where invertable and non-invertable circuits may be nearby. Consider, for example, different circuits running through the same conduit, outlets which have invertable and non-invertable circuits running to different recepticles, etc.
As a result, the metal case of the inverter needs to be tied to effective equipment grouding conductor (EGC) for each of these circuits. The only way to achieve this is by bonding the two.
I have also wondered why the AC GRD and DC GRD are bonded providing a docked boat a return path to shore through water.
This is one that gets talked about a lot.
I think the most straight-forward example here is to consider a short from an 120VAC hot wire to anything electrically tied to the boat's DC negative, e.g. the metal chassis of DC devices like radios, the engines, the bonding system, etc.
This scenario, by itself, won't trip any over-current protection devices, either aboard or on shore. It just puts a high AC voltage potential on all of these devices.
Now imagine that someone touches any of this metal and anything tied to a groundin conductor, neutral conductor, or earth. This could be someone within the boat -- or a swimmer or diver. Zaaaaaaaap (especially for a freshwater swimmer or diver, this might well mean electrocution facilitated paralysis and drowning)!
As for those who complain that this arrangement might facilitate galvanic corrosion -- they are correct. No denying it. This is why a galvanic isolator or isolation transformer is important in a modern boat. We traded human safety for the safety of metal from corrosion -- and the galvanic isolator or isolation transformer help us reclaim the safety of the metal from corrosion.
Some folks ask why we need to do this if ELCIs, GFCIs, RCDs, etc are present and should protect people from harm in such circumstances. Well, first, these devices aren't universal, second having a belt and suspenders is often helpful, 3rd, adoption of such devices is being slowed by those with non-compliant, leaky electrical systems, e.g. those without separate neutral and grouding conductor buses, and finally, who trusts marina electrical distribution to be up-to-date and well maintained?