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A single 30A 120V shorepower inlet would be sufficient for a small-to-medium sized boat. Larger boats use either two 30A inlets or a single 50A 120/240V inlet. The big megayachts use one or more 100A inlets, or even larger 3-phase systems.
Standard shorepower cords are sized for the appropriate amperage - longer cords use heavier gauge wire, shorter ones use smaller.
Remember that the amperage ratings are continuous. Startup surges for AC induction motors can be two or three times the continuous draw, but typically don't last long enough to trip a breaker - unless there are other loads running.
What size air conditioner have you seen that draws 30A? I have two MarineAir units - one's 16k btu, the other's 12k btu. They draw a little less than 13 and 15 amps, respectively. The minimum recommended breakers are 24 and 26 amps. One's on one 30A shorepower circuit, the other's on a second. I do have to be careful about running other high-current draw appliances, like the battery chargers or the hot-water heater simultaneously. But hey - it's a boat, not a house! You have to be more conscious about power consumption, anyway - especially away from the dock!
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