NTSB report [As summarized on Wikapedia}

On July 25, 2006, the final report for the Ethan Allen boating accident was released by The National Transportation Safety Board.[1] It established that Ethan Allen had capsized as a result of insufficient stability. The US Coast Guard was responsible for the original certification of the vessel, then called the Double Dolphin. It was thought that the original certification of 48 was allowed due to different test standards applied in 1966; passenger weight difference was not responsible for those differences.

When the owner of the Double Dolphin fitted the boat with an elaborate pipe structure and canvas top, the center of gravity was raised and, more importantly, the projected side area vastly increased. The boat continued to be inspected and certified by the USCG for 48 persons, although they should have performed another test. According to the NTSB Naval Architect, Rob Henry, if that test had been performed, the boat would not have been certified to carry any passengers due to insufficient stability to the load of the wind heel.

In the mid 1970s the boat along with two other sister vessels were sold after a company insolvency to Shoreline Cruises in Lake George. The US Coast Guard certificates were at that point still current and valid. New York State vessel regulators were given those certificates and, based on the US Coast Guard officially stating (incorrectly) that the boat had sufficient reserve stability to safely carry 48 passengers and 2 crew, New York regulators then gave the boat the same rating. In 1989, when the owner had a hard (wood and fiberglass) top installed to replace the pipe structure canopy, it was determined that its lower height more than compensated for its slightly greater weight, and vessel stability was increased to the point that passenger loading would have gone from 0 with the former top to 14 passengers. Because of the increase of stability, state regulators did not require a new stability test of the vessel, and the rating continued at 50 (48 passengers and 2 crew members).

According to the NTSB, the capsizing occurred due to a series of events. The total load was over three times greater than it should have been, the pilot made a hard turn to the right at speed, the seating arrangement put three people on the left versus two on the right (giving a permanent two-degree list to port), and the bench seats had no way of stopping involuntary sliding, so the centrifugal force of the hard turn caused involuntary movement of the passengers to the port. That movement increased the list to port and the increased list caused more sliding. Within seconds the center of gravity was no longer over the hull and the boat rolled over.

It was also confirmed that the number of passengers (47) was too many and that the boat was only capable of safely carrying 14 passengers.