So it was practised for teaching swimming here in Poland at schools, but these are always in a very shallow pool and supervised too. They lack the failure mode

So it was practised for teaching swimming here in Poland at schools, but these are always in a very shallow pool and supervised too. They lack the failure mode of floating face down in water, but instead are unsafe to others who can be hit with them. And kids will collide. The newer foam ones are much softer and safer.