Reads like the steering cable broke and without a kill switch tether, it circled back on them.
As a little girl, I was riding in a boat-pulling a skier that my brother was kneeling on the seat driving and that steering cable broke. My brother got thrown out in a perfect dive and the boat started circling. Skier dropped off and I (being little and not knowing any better) tried to stop the boat by pushing the wrong way on the gas lever-cirling faster then. But finally thought to turn off the key. Luckily my brother surfaced unhurt. Sorry this accident turned out differently.