How low? Not 16 feet above the water? Safe altitude 2,500 feet above the water? Operate mostly single pilot no co-pilot? DHC-3 Otter has autopilot? Did any similar incident happened before with float planes flying in the Pugget Sound?
If you look at the top of the article at the link I posted there is a flight/windshear profile from Flightradar24 at the location and time of the flight.
The plane was at about 1000 ft and dropped at exactly the point a strong windshear was occurring.
That could just be a coincidence I suppose. If you have any evidence the pilot had a heart attack or stroke, please share.
Everything is speculation right now.