Having over 6000 hrs in Beavers on Floats, a few years in hangers in the states maintaining them, an ex chief pilot in Sydney well before Sydney Seaplanes and then obviously been involved in the Cottage Point lunch flights many times, I am at loss to put any credence on the Carbon Monoxide theory. The construction of a Beaver has a very loose cowl fitting which gives an approximate fist size gap around approx 90% of the cowl and the airframe. All Beaver pilots I have flown with taxi with the pilot window down to keep fresh cool air circulating in the cabin. In most cases the pax window is down also. Being the time of day and the time of year I suspect both windows would have been wide open. With the take of direction the aircraft would have been taxied into wind with the occasional turn cross stream so as to avoid taxiing down wind. Down wind Taxi on water destroys propellers. There may have been a 360 turn or two involved so as not to extend the departure point too far east., but in any case there would be a very significant flow of fresh air in the cabin. A few holes in the firewall to allow CO into the cabin in sufficient volume to affect the pilot and passengers to me sounds extremely unlikely. BR