Next to breathing, your vision is also important underwater. The human eye is not able to fully focus underwater. This is why you need a dive mask to enhance your vision and focus so you can see clearly underwater as if you were on land. It is designed to provide an air pocket that prevents any water from coming into contact with your eyes.

When the human eye comes in direct contact with water, the light entering the eyes refract by a different angle. This makes the eye unable to focus the light. By providing the needed air spaces via the air pockets in front of the eyes while underwater, light is then able to enter and focus normally.