All speculation but the weather conditions were there for windshear as Cliff Mass says at the end of his post. Also, the plane was actually climbing at the time of the crash from 600 to 1000 ft. and losing speed. So, if a windshear happened at just the wrong time during a climb...bad luck.
Or maybe not. That’s why the NTSB is there.
I don't think so? I replied you the FlightAware transponder's data, the float plane was at 900' at 144MPH then at last seconds pilot trimmed throttle down to 115MPH as if intended to do emergency landing in the water but still at 900'? Your 2013 Youtube video is irrelevant, you would need to fight out then current weather observation at Pugget Sound or even Sea-Tac weather observations around the area. Witnesses actually saw the plane plunged down in to the water at 45° dive. The stupid Covid-19 injection you call it vaccine will also cause vertigo, or pilot just had cardiac arrest, physically incapacitated, no chance to declare emergency, no may day.