I was wondering where I should start.
1st be extra cautious and do not use your AC system until this is sorted out. 2nd as mentioned get yourself a device that will test an outlet for reverse polarity, like this
link. Since this style is made for typical outlets, and your dock’s power pedestal / boats power cord is probably 30A also get yourself a converter like this
link. Alternatively, they do make 30A polarity testers. 3rd using you new reverse polarity tester plug it into the pedestal using the same outlet that you did when you got the A/C Panel’s reverser polarity light. If not good contact your Marina.
If all is good, then evaluate your cords between the pedestal and boat connection. Is it a fully molded cord without any tears or repairs? If yes, its still possible an issue. Use your new tester at the end of the cord as well. Any options to replace it or perhaps borrow one? If cord is good and still getting reverse light, unplug and visually inspect behind the A/C panel and in around the ckt breakers. Any noticeable corrosion or frayed wiring? If yes and you are comfortable with repairing, do so. If not comfortable or no noticeable issues, suggest contracting a marine certified electrician.