Abstract: Arrangement with an acoustic array with a sound velocity meter, suitable to be arranged under water. The acoustic array is able to receive acoustic signals from an underwater device and to provide acoustic array output data to a processing arrangement such that the processing arrangement can perform in real-time, a calculation of a position of the underwater device relative to the acoustic array based on the acoustic array output data. The sound velocity meter is able to measure velocity of sound in fluid layers just below a vessel floating on the water and to provide sound velocity meter output data to the processing arrangement such that the processing arrangement can correct the calculation of the position of the underwater device based on the sound velocity meter output data in real-time.