If people know they have to go through this ceremony then it doesn't have to be a near-death experience at all. They simply need to practice holding their breath to be many times better than average and make sure that guys pulling that rope won't decide to kill them.
Historically, It's considered to be one of the most severe
forms of punishment in marinist circles. I know a few sailors and they sometimes jokingly refer back to this when you do something wrong on the boat/yacht. "Hey don't do that or we'll have to drag you under the keel" and the like. So it still has some cultural retention.
I think the significance of this ceremony is that an individual puts faith (life) in the community for a short time. It's a symbolic act of becoming a part of some sort of group.