Water has a higher refractive index than air. Light entering the eye from the water behaves differently than light entering from air. This creates a distortion that affects normal vision by causing very severe hypermetropia. That is why people with severe myopia can see better underwater without a mask than normal-sighted people.

Diving masks and diving helmets and fullface masks solve this problem by creating an air space in front of the diver's eyes. The refraction error created by the water is corrected as the light travels from water to air, except that objects seem to be nearer than they are.