Electromagnetic (EM) flow control of boundary layers refers to the use of ‘wall-flush’ electrodes (j, current density) and ‘sub-surface’ magnets (B, magnetic induction) used in combination to create local Lorentz body forces (j × B). In the present application the working fluid is seawater. Close to the boundary wall, these j × B forces can act directly on velocity and vorticity. In this paper, the characterization of a wall-normal EM actuator (i.e. j × B forces are mainly wall-normal above the central axis of the actuator) is considered. An idealized inertial and integral approach leads to the definition of characteristic EM numbers in term of velocity, time, acceleration and length-scales. These numbers are useful in introducing an EM parameter similar to the Froude number. Furthermore, two asymptotic EM flow regimes, which depend on flow velocity and on EM force intensity, are also discussed.