  We discuss the generalisation of the so-called traditional approximation,
well known in geophysics, to general relativity. We show that the approximation
is applicable for rotating relativistic stars provided that one focuses on
relatively thin radial shells. This means that the framework can be used to
study waves in neutron star oceans. We demonstrate that, once the effects of
the relativistic frame-dragging are accounted for, the angular problem reduces
to Laplace's tidal equation. We derive the dispersion relation for various
classes of waves in a neutron star ocean and show that the combined effects of
the frame-dragging and the gravitational redshift typically lower the frequency
of a mode by about 20%.
