It doesn't look like there are 500 people on board the Mt. Washington II just after the launch.
Perhaps it is an urban legend?
Wow! Look at the crowd and the boats!
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An excerpt from Lake Winnipesaukee Historical Society
The M/S Mount Washington
To the boys of the town, the perfect fillip to the day came when the boat, for which a six-inch clearance had been expected under the Weirs bridge, had to stop, and they were invited to jump on to provide ballast. It took but a few minutes to get the necessary "ballast," the flags were put on again and then the boat received a tumultuous welcome at the home dock at Weirs.