Here we see the most common type of Waterproof Crown; Which is commonly called the exposed O-Ring Crown. The exposed gasket that you see can, and normally does need, changed during the average service or over an extended period of time. The Gaskets do dry out and often break and replacing these will ensure proper maintenance of your Waterproof Watch or timepiece.
This is another type of Waterproof Crown and it is commonly referred to as a "Hidden Gasket" Screw Down Watch Crown. This type is very difficult to change the Gasket and most times it is recommended and necessary to change the Crown completely.