In other reports the daugther says the speed was 10 to 15 knots, but equated that to about 30 mph. I think the 30 mph comment was then picked up by other new casts - 15 knots is about 17 mph.
If your dinghy was hit by a 30 mph speedboat, pretty sure that would be it. Not to mention 30 mph in an anchorage is almost murder.
However we know very little about this terrible accident. Hope it works out OK. Wonder what happened to the speedboat driver, the occupants noticed they were dragging the tender and person, they must of then stopped and taken him to shore?

J&B