It is not uncommon when diving to have a little water leak into your diving mask. This can happen if there is a small breach between the seal and your face (for instance you get hair between them). Sometimes your mask can get flooded if it gets knocked for whatever reason.

There is absolutely no reason to panic if your mask leaks or gets flooded. It is quite possible to open your eyes underwater with no discomfort (for some people it just takes some getting used to). If your mask is flooded it can obviously have an effect on your vision, so it's nice to clear it. Sometimes you'll even flood your mask intentionally to clear a foggy lens!
Steps to clear your mask of water
1) Inhale through your mouth piece and get a decent lung full of air
2) Hold the top ridge of your mask firmly to your forehead with your fingers
3) While still holding the top of your mask breath out hard through your nose
4) Tip your head back slightly so you are looking diagonally up while continuing to exhale through your nose

Air will now fill your mask and push the water out. Hey-presto! You're water free again.