This is kind of complicated to explain
my other excuse is I am a lousy teacher so I will refer you to this site.
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!").