Reverse polarity means the hot and neutral wires have swapped places somewhere between the dock pedestal and the boat's AC panel. Everything still works, so nobody notices. The danger is what stops working: breakers interrupt the hot wire, so with polarity reversed a single-pole breaker switches off the neutral and leaves the circuit live.
ABYC E-11 asks 120 V shore power systems for a reverse polarity indicator, a light or alarm that signals continuously when polarity is reversed.
What a surveyor checks
With shore power connected, look at the AC panel for the indicator and whether it's lit. Then test: a plug-in polarity tester at the dock outlet tells you whether the fault is ashore, and the same test through the cord and at the boat's inlet narrows it down. A meter between neutral and ground should read close to zero.
In the report
"AC panel, no reverse polarity indicator fitted. Polarity correct at time of survey. Fit an indicator to ABYC E-11 so a reversed supply is seen before the boat is used." See shore power reverse polarity.