A device for measuring physical activity of a user during swimming is provided, which includes the following: a sensor unit adapted and dimensioned to be coupled with swimming goggles, wherein the sensor unit includes a motion circuitry configured to measure physical motion of the user; an optical cardiac activity circuitry configured to be placed at least partially against a body tissue of the user and to measure cardiac activity of the user; and a wireless communication circuitry configured to transfer data between the device and at least one external device, wherein the wireless communication circuitry is configured to transfer physical activity data of the user to the at least one external device, and the physical activity data includes at least one of motion data obtained using the motion circuitry, cardiac activity data obtained using the cardiac activity circuitry.