Abstract: A sonar buoy 40 has narrow-beam sonar heads 80, 82 driven by a stepper motor 84. The heads produce sonar data for example accurately measuring the seabed. The buoy is handled by a crane wire 41 held taut by a float above the buoy and attached to a clump weight 46 resting on the seabed (although shown in retracted condition). A wire 50 runs from a winch 48 around pulleys 52 on the weight 46 and is connected to a lever 54 of a gimbal mechanism 56 which has two transducers 62, 70 giving measurements of the "X" position and the "Y" position of the buoy. A footage mechanism 76 is driven by the wire 41 and has a transducer giving a measurement of the heave position. A compass 88 and roll and pitch transducers at 86 give measurements of the heading position and the roll and pitch positions. The sonar data is compensated for the position of the buoy using a computer.