The class was held in the school swimming pool, which had a max depth of 18 ft, as I recall.
One time I forgot the "continue to breathe" rule (briefly) and even after ascending only five or six feet it got exceedingly uncomfortable (at which point I remembered "Oh yeah - BREATHE, DUMMY - AND DON'T DO THAT AGAIN!").
I'm glad you are safe.
I've seen 1 young man die. It was heart breaking and unpleasant.