In a method for determining absolute position under water of a submersible vessel (1) having a dead reckoning navigation system and receiving acoustic signals from a reference station (19), signals are received from one reference station in several positions (15 18) of the vessel. Estimated absolute positions of the vessel are calculated using range data and relative position data. Range rate data derived from the signals are preferably utilised. In a method for scanning an underwater survey area, the absolute position of a vessel (1) is intermittently being determined according to said method. The reference station may be placed at a fixed absolute position (19), or on the surface of the water, preferably in a buoy or a vessel. A system for determining the absolute position under water of a vessel comprises: acoustic communication means in a reference station and on board the vessel; a dead reckoning navigation system on board the vessel; and computing means.