Watchmakers generally indicate the water resistance of a watch in meters (ie: 30m, 50m, 100m, 200m and so on…). The consensus is that a column of 10 meters of water creates as much pressure as the Earth’s atmosphere; so when the watches are tested in a lab and put under atmospheric pressure to determine their resistance, the result is expressed in Atmosphere or Bar, and then multiplied by 10 to indicate a theoretical water depth.
The problem with this depth measure is that it only corresponds to static depths. On the field, a swimmer’s movements and the water turbulences will expose will add to the pressure the watch undergoes. Seiko has compiled a more accurate table to help chose a watch based on the level of exposure to water: 
For more reading, we recommend the excellent article on water resitance published on Quartzimodo.com
