In this work an algorithm for on-line estimation of the hydrodynamics in underwater vehicles is presented. The identification comprehends matrices related to the added mass, the linear and non-linear drag, and the Coriolis and centripetal terms. It is required the measure of the thruster and cable forces and the velocity of the vehicle. Identifiability and convergence of the parameter trajectories are analyzed. The nonlinear drag is approximated by skew symmetric basis functions of the velocity components to a high degree of precision. A case study of a spherical ROV shows the features of the algorithm by numerical simulations.