The shot is too tight to really see what was going on around him. But it *looks* as if he was turning into a wake (you see the bow os his boat bounce a little) and then suddenly submerged. Given that it looks like some kind of a wake boat, I'm guessing as well that the sudden dip in speed caused ballast weight to shift forward and push the bow down into a trough of a wake, which then came over the bow.
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