Ever
I did yesterday. For a moment. In that moment my 5 yo let go of the float and slipped under the water. He was out of his depth.
He was only under for a few seconds but when he was scooped out was still. I was so frightened but he was breathing thank god. He didn't cough or splutter, he was very very quiet. He was taken out, wrapped up and given a big cuddle. He was running around playing ten minutes later but since then I've been reading up on secondary or dry drowning and feeling like the Worst mum in the world 
It happened so quickly. He is ok and I'm ok, but I am still feeling shocked at what could have happened.
I thought I was vigilant, but I took my eyes off him for literally seconds and that was all it took.