The Rolex Oyster waterproof case was patented in 1926.
The Oyster addressed a major weakness of early wristwatches: dust and moisture could enter around the crown, crystal, and case joints, damaging the movement. Its hermetically sealed construction used a screw-down case back, bezel, and winding crown to protect the mechanism. Rolex introduced the design as the Oyster in 1926.
There is a subtle historical detail behind the date. Rolex acquired an earlier screw-down crown patent associated with Paul Perregaux and Georges Peret; that patent had been filed in 1925 and published or granted in 1926. Hans Wilsdorf then developed and promoted an improved Oyster system, with related patent work continuing into 1926 and 1927.
Rolex publicized the case dramatically. In 1927, swimmer Mercedes Gleitze wore an Oyster during an attempted English Channel crossing. Although she did not complete the swim, the watch survived hours in cold salt water, giving Rolex a powerful demonstration of its water resistance.