The metre (or meter) sea water (msw) is a metric unit of pressure used in underwater diving.

[1][2] The unit used in the US is the foot sea water (fsw), based on standard gravity and a sea-water density of 64 lb/ft3.

Atmospheric pressure may be considered constant at sea level, and minor fluctuations caused by the weather are usually ignored.

[8] In metric units the ambient pressure is usually measured in metres sea water (msw), and converted to bar for calculations.

Feet and metres sea water are convenient measures which approximate closely to depth and are intuitively simple to grasp for the diver, compared to the options of more conventional units of pressure which give no direct indication of depth.