This is a great image illustrating all the gaskets that go into a Submariner and the GMTII-C, Yachtmaster, and any other Rolex that sports a TripLock crown. This image was taken by Jordan P. Ficklin, CW21, and is posted on his WatchMaking Blog.
It is good to have your watch pressure tested every year to check these seals. If the seals fail, replacing some of the seals (eg. crystal/bezel) appear to be fairly involved.
Click image for larger photo. Photo used with permission.