Very interesting Ian, I hadn't seen the article, but I've just read a few of the stories in the papers.
The Wandsworth police turned the case over to Scotland Yard who presented findings to the CPS who then instructed them to charge the guy with Manslaughter by Gross Negligence, then a District judge sent the case to the Old Bailey.
BUT would it mean that I would be on a similar charge if the other half fell over board, I hit something underwater, had a collision day or night and something happened? Im not suggestion boating immunity just because he owned a boat but it makes me to think that if one thing goes wrong (mayday or pan pan ) and Im in the dock possibly for years. Or would he be let off IF they were wearing jackets? even so one of them if not both possibly would still have died in that stretch of river in December, freezing and severe under currents etc.
I just looked up Gross Negligence in English law and some articles say that it is only in recent years that the law has distinguished between "negligence" and "gross negligence". I'm not sure if this statement about the law answers your question or not but .................
“gross negligence” was clearly intended to represent something more fundamental than failure to exercise proper skill and/or care constituting negligence.
The concept of gross negligence is capable of embracing not only conduct undertaken with actual appreciation of the risks involved, but also serious disregard of or indifference to an obvious risk. On the face of it I think this is why the District Judge referred the case to the high court in order that he doesn't have to interpret the law. However this guy has been charged with Gross Negligence Manslaughter, a couple of extracts here....
Gross negligence manslaughter is a form of involuntary manslaughter where the defendant is ostensibly acting lawfully
Gross negligence manslaughter is not dependant on demonstrating an unlawful act has been committed
Gross negligence manslaughter can be said to apply where the defendant commits a lawful act in such a way as to render the actions criminal I think therefore your question is valid, but In answer to it, I haven't got a clue. However interpreting what you said about freezing water and life jackets is an "indifference to an obvious risk" as above.