First things first: Thoughts and prayers for all those that lost loved ones in this tragedy.
From the video by the way it was sitting so low in the water just before she capsized (which wasn’t shown) it sure looks like he got swamped. Those DUKW’s are so heavy with a set of wheels and axles sticking down into the water flotation is already an issue. If there wasn’t a bilge pump and/or if it wasn’t functional or running that would have been a major contributing factor.
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It rode much lower than the other DUKW, so my guess is that it’s bilge pump was not operational. Once it got full of water it was going down.