If it's as the original poster describes, this method might work for some children but one doesn't necessarily know how it would affect any particular child. The description makes it sound pretty harsh -- not what I would submit any child to.
I did check out ISR's web site, and it paints a different picture. They say they monitor the child's comfort level and tailor the lessons accordingly. I watched a short video and if this is indeed how they do it, I would consider it. And it appears that a parent is there during the lesson. If it were my grandchildren, I might want to observe one or more lessons.
The funny thing is that when I first heard the name "Total Immersion", I thought it was something like this -- being thrown into a deep end to bring forth instinctive "swimming".