I presume a dive shop advised how to select your mask, (push lightly onto face - breathe in slightly and
slowly. Mask should stay on?).
Boys can have trouble with mask leakage if let a moustache overgrow.
It's easy for girls to have some of their
flowing locks stuck under the mask's edge or skirt.
A swimming cap under the edge of the mask can do it. Thats all it can take.
So, obviously, first
check for anything as simple as that.
It might seem like the thing to do but tightening the strap can make things worse. An overtight
mask creases your face which lets water in.
So, best not to share a well-fitting mask, although it can seem mean.
Usually it's a matter of surfacing, checking for an obstruction (hair, cap etc.) then simply fiddling by
repositioning the mask slightly.
Often you never really know what was wrong but if it's dry in that mask again then don't worry about it -
focus on the fish.
For a small, slow but annoying leak, raise your head slightly above the vertical so water is at the
bottom of the mask near your nose and gently breathe out.
Blow hard and saltwater splashes into your eyes.
You can do this easily and quickly. No need to surface. The noise and bubbles can
surprise the fish though and scare them off.