It seems a bit silly to me and counterproductive - i'm sure if the horse was that sure it wanted to buck, water wouldn't stop it. I would work on ensuring the horse is confident and ready for it's first ride than tricks used to stop it bucking or lessen injury!
I've always heard the old cowboys that break their horses in the water. The thing that always got me. Is if the horse isn't waterloving. Then your practically going to have to drag it out there unless you have a decent bond and partnership. And if you have that then getting in the saddle the first time would be a pretty easy process. You would also end up with a wet saddle and that's not very fun if it's leather!! Trust me on that one!
I thought it was an old Native American technique. They were supposed to be great horsemen, apparantly. Hard to know now what is myth and what was reality about all that.
It worked for Alec in The Black Stallion, lol!