The man was effectively acting as the instructor/supervisor.
If you speed or are caught driving dangerously in a car as a learner whilst under the supervision of a qualified driver, guess who can also get the blame...*
Or if you let someone operate heavy machinery in a factory without giving them the training and supervision to ensure they are safe to operate it (let alone if you are standing next to them allowing them to use it in a manner you know is dangerous but they may not realise) - it's one of the reasons most companies document who is trained to use potentially dangerous equipment and will stop people who are using it in an unsafe manner when they become aware of it (or at least should do if they don't want to be left open to prosecution under H&S laws, or at the minimum find insurance much harder to get).
As the person who owned or hired the boat and who let a completely inexperienced person use it, he had a responsibility to monitor and supervise it.
The fact he'd got a record of breaking the law/acting like a pratt in boats in the past and had been warned about his behaviour would have weighed quite heavily in the eyes of the law, and almost certainly the jury, as it establishes a pattern of irresponsible behaviour in the boat and a disregard for the safety of others.
Oddly enough in most areas of the law if you've got a record of doing something repeatedly despite being warned by officials that you were breaking the law/putting people in danger, the law tends to be a lot harsher on you when someone does get hurt.
*I have vague memories of my instructor (going back over 20 years) mentioning the fact that one of his friends had got points/a telling off by a traffic cop because he'd not noticed his pupil had gone over the limit during a lesson.
