Abstract: An apparatus, a system, and method for generating underwater navigational information includes receiving data from a user, obtaining inertial data from inertial sensors, producing an estimation of a propulsion power by the user at a predetermined points of time by calculations at least partly based on the obtained inertial data, a hydrodynamic coefficient is calculated for the user at least partly based on the input data by the user, a velocity of the user is calculated at least partly on a basis of the estimated propulsion power and the determined hydrodynamic coefficient, heading information of the user at the predetermined points of time is calculated from the obtained inertial data, the calculated velocity information is combined with the calculated heading information at corresponding points of time and navigational information is generated.