A method comprises transmitting (901) a first ultrasonic ping signal from a first floating unit (M1). The first ultrasonic ping signal is received in an underwater device (D1, D2, D3). After a predetermined delay from the reception of the first ultrasonic ping signal, the underwater device transmits (905) a second ultra- sonic ping signal from the underwater device. The second ultrasonic ping signal is received in the first floating unit. The first floating unit determines a time difference between a time of transmission of the first ultrasonic ping signal from the first floating unit and a time of reception of the second ultrasonic ping signal, and based on the time difference, provides location information and/or other information to the underwater device by transmitting (910) to the underwater device sequential underwater ultrasonic ping signals such that time differences between the sequential underwater ultrasonic ping signals indicate the provided information.