A "proper" light duty AC system would mean a shore power cable, a ground fault interrupt (GFI) safety device, adding an AC breaker to the panel, and running some AC wiring and receptacles on the boat. Couple of hundred bucks and one long day or casual weekend to install it all if you are familiar with it. More expensive if you are hiring labor.
Well that was 9/10ths.... but you're right. And this is
a good question.
You can, and should, connect the AC system on your boat to the boat's bonding, or grounding system. However, if you do, the AC ground will provide a direct electrical connection between the boat's ground, and anything else directly connected to the ground wire (other boats, the dock, etc.). This creates a great environment for galvanic corrosion. In addition, if you use a single pole breaker (like most homes), in the case of a shore power fault (reverse polarity - more common than many realize, or two prong power cord incorrectly inserted), the entire
ground system (AC and DC) could become hot, causing a hazard for people on board, and in the water!
Thus: a proper AC system on a boat should include two pole circuit breakers
and, optionally, a isolation transformer.