Final answer:
A dry contact lens has different power compared to when it is on the tear layer due to changes in the refractive index. Underwater, vision is blurry because of the difference in refractive power between water and air; a face mask provides an air space for clear vision.
Step-by-step explanation:
When a dry contact lens is not placed on the eye, its power is different than when it is resting on the tear layer of the eye. This is because the refractive index changes when the lens is in contact with the tears. The tears create a conductive environment that allows the contact lens to work properly with the eye's own refractive elements, optimizing its corrective power.
The reason why your vision is blurry when you open your eyes while swimming underwater is due to the refractive power of water being different from air, which our eyes are adapted to. Without a medium such as air or a face mask, light rays do not focus properly on the retina. A face mask introduces an air space in front of the eyes and returns the refractive capabilities closer to what we experience in the air, allowing for clear vision.