This situation interests me, thank you Chronus for posting this.
A few clarifications: In Canada, the only person who can "provide" a seaplane rating is a Transport Canada "Authorized Person", a privilege held by a few instructors. My CPL was signed into my license [booklet] by an authorized person, who was the Chief Flying instructor at the time. However, a CPL, who meets the minimum experience requirements, can "recommend" a trainee for a seaplane rating, having followed the required course of training. The recommendation is to an "Authorized person", and the rating will be issued (everything in order assumed). I have provided such training. In this sense, yes, the pilot goes from zero skill to rating, without having flown with an instructor. Both my multi engine, and seaplane ratings were achieved this way. This probably harkens back to the day when there were lots of "bush pilots" out there who had the skills, but too few instructors to formally train new pilots in those skills. TC found a way to harness the pilot skill of non instructors.
In the case of the event described, assuming Canadian pilot licenses were held as required, the situation probably met the licensing and training requirements, if not those for safe piloting skill.
The more comprehensive text is found here:
Seaplane pilot found guilty of manslaughter - Newsletters - International Law Office
There are details which make it worth the read, some very subjective things in there. I presume that the court found its way to the fact that in Canada, one does not have to be an "instructor" to provide flight training for a sea rating. I can imagine this one going either way based upon the understanding that the "student" had as to the role played by the training pilot. Was the training pilot that, or a misrepresented "instructor".
But, in any case, this does raise very valid issues about applying training pilot skill during instruction. As a training pilot, I have found myself having to stay several steps ahead of the mistakes being made by my charge while flying. In the very most serious, amphibian training, and allowing [either pilot] to land on the water with the wheels extended. In any case, an engine failure/power loss is much more risky in floatplanes, they glide to a landing okay, but more aggressive nose down entry to the glide is required, and effective resistance to the "water rush" which temps one to pull up much too early to flare.
In my opinion, the Swiss court found the hard line on this, but I can't say they were wrong, we have a duty of care....