The movement of the wearer's arm through the water increases the pressure on the watch dramatically. Over time chemicals may deteriorate the gaskets and sealants. Hot air, sun and colder water cause expansion and shrinking, the 0 rings may be deformed. If the watch is a chronograph, the chronograph pushers will also have O rings. Cases are lined with a sealant, applied in the form of a hardening liquid, which helps keep water out.
Some watches use special silicones as sealant.
The thickness and material of the case being sturdy enough to withstand water pressure without caving in. Withstanding splashes of water during washing hands, rain. Should not be submerged. Made for deep sea diving with gaskets withstanding the helium used in decompression chambers. Possibly with over-pressure valves that let release the helium that has seeped into the watch when surfacing, so as to avoid that the watch case explodes.