Watches may be crafted to become water resistant. These watches are sometimes called diving watches. The International Organization for Standardization issued a standard for water resistant watches which also prohibits the term "waterproof" to be used with watches, which many countries have adopted. Water resistance is achieved by the gaskets which form a watertight seal, used in conjunction with a sealant applied on the case to help keep water out. The material of the case must also be tested in order to pass as water resistant.

The watches are tested in still water, thus a watch with a 50 meter rating will be water resistant if it is stationary and under 50 meters of still water. For normal use, the ratings must then be translated from the pressure the watch can withstand to take into account the extra pressure generated by motion. Watches are classified by its degree of water resistance, which roughly translates to the following:

" Water resistant - Will tolerate splashes of water or rain

" 50 meter - Usable while showering, bathing, dishwashing, and swimming in shallow water

" 100 meter - Usable while swimming, and snorkeling

" 150 meter - Usable during general water sports

" 200 meter - Usable during general water sports, including free diving

" Diver's 150 meter - ISO standard for scuba diving

Some watches use bar instead of meters, which may then be multiplied by 10 to be approximately equal to the rating based on meters. Therefore, a 10 bar watch is equivalent to a 100 meter watch. Some watches are rated in atmospheres (atm), which are roughly equivalent to bar.