When I was younger (say, 7 or 8 years old), I considered myself a good swimmer and as part of my swimming lessons we were told to do length-ways with the aid of a swimming board [0] at arms' length.

Approaching the 3/4 mark of the length, I felt it slipping out of my hands and in my panic, I went down. I woke up after being resuscitated after a lifeguard noticed I was in trouble under the water. He had jumped from the top step of his high chair straight in apparently and literally threw me out of the water to a waiting colleague.

Not sure if I'd trust my own children with that type of flotation aid again, but as you said, it encouraged me to swim outside of my comfort zone and I panicked so perhaps more control is needed of when and where it is used.

[0] http://runforthethrills.com/wp-content/uploads/2013/10/kick-...