This is a Rolex cyclops magnifying lens found on the crystal of various sport models. The watch shown is an X series Submariner date model 16610. Magnification level is 2.5x. The magnifier material is sapphire. It is possible to get a Submariner without the cyclops (referred to as a No-Date Sub) as well as possible to remove the lens itself.
Authenticity Guide: This series of videos can be used as a basis of comparison to help determine if your watch is real vs fake.