A description is given of a method of measuring the velocity of sea-water, using the fundamental law of electrodynamics, regarding the water as a conductor and measuring the e.m.f. generated when it moves relative to a magnetic field.Descriptions are given of an underwater unit for producing the magnetic field, apparatus for measuring the motive e.m.f. and experiments made with the unit in a flume in order to calibrate it. It is shown that the method is very convenient for comparing velocities and can be used for the measurement of water velocities due to swell, tidal current, etc.