Some boats have the generator fuel pickup set above the engine pickup. The generator will stop operating usually somewhere between 25% to 33% fuel remaining leaving you fuel to run the main engine. I suspect perhaps that is what he means.
On the subject of crowds in June it actually is about the 2 nd busiest time of year now from about 10 June to 1 Jul. The reason is family vacations when the kids get out of school. The first week of June can be deserted!
G