Masks

The human eye is not designed to work in water, just try opening your eyes in the pool; everything is blurry.  This is because the refractive index of water is different to air, and our eyes have evolved for air, not water; so you have to wear a mask to be able to see clearly.  Scuba masks must have a toughened glass lens, to be able to resist the pressure underwater, and are available with single (one piece) lens or multiple lenses (two or more).