Water resistance indicators happen to be quite often mistakenly interpreted, this, therefore, causes unnecessary damage to the watch. We indicate water resistance in metres, atmospheres or bars. These units indicate the level of pressure applied to the watch caused by the water column.
So, as determined in our table of water resistance, the pressure of 30M (metres) equals the pressure of 3 atmospheres. Watch with this level of water resistance can survive the higher level of condensation and basic contact with water (e.g. summer rain), but is definitely not proper for swimming. If you plan to have a bath while wearing your watch, or swim in the pool, aim at the watch with 10 ATM/ 10 BAR indicators.
In the case of diving, the resistance level of 20 ATM/ 20 BAR or 30 ATM/ 30BAR might come helpful.
One of the determinants is whether the water's salty or not - it's highly recommended to carefully wash your watch after swimming in salty / highly chlorinated water. You should also keep in mind that the water resistance is measured in the laboratory (e.g. ideal) conditions.