I am aware of the small boat channel. This boat was not following the rules which say he should keep to the starboard side of the channel, which is fairly wide in relation to a yacht. If he had done this he would have had a bumpy ride but nothing more than that. As it happened, he ended up grinding up and down the wall. "Got a new gel coat, guv'nor?"
You are right about the wash, it is 'normal' and it is probably because of this I have seen small boats waiting outside the entrance for the extra 2 minutes it takes for a large vessel to pass.
As for the bloke at Watchet, whenever I have taken a small boat out of a harbour like that, where the entrance is obscured from view, I have taken the boat away from the entrance initially. When some distance from the entrance, I would then turn so the entrance is clearly in view, with a healthy distance in which to decide whether or not it would be safe to continue. This bloke just charged straight across the entrance as if he was the only vessel in the world.
If the yacht had started his move just a couple of seconds later, the outcome would have been radically different.