30m is low rating of water resitance. It is sometimes known as "splash proof". It means your watch is safe for incidental contact with water and liquid but not for submersion.
5 ATM / 50m
Suitable for submerging. Can be worn whilst swimming for shorter periods of time but possibly not suitable for jumping in to water. Suitable to wear whilst showering. We do not recommend that you wear leather straps for swimming.
Suitable for swimming and diving at shallower depths
Suitable for swimming and diving (but not deep sea diving).