Entries and exits are where masks get lost. Usually because the owner takes it off their face. Underwater is usually not a problem, even a broken mask strap isn't going to make your mask fly off your face. I have my students pull the strap off their head, but leave the mask in place. They then exhale through their nose and shake their head back and forth to try to get the mask off without using hands. It is pretty rare that students are able to do it and it takes so long and so much effort that it is clear that masks don't just pop off underwater. The pressure difference does a good job keeping it on your face. I usually do this when I can't convince the divers that their mask straps are entirely too tight.
-Chris
-Chris