In this paper we study a special mode that is associated with the ocean/crust boundary in neutron stars (NSs), investigating its properties both analytically and numerically. We find that this mode acts like a shallow surface ocean wave, but with a large radial displacement at the ocean/crust boundary due to flexing of the crust. This displacement lowers the mode's frequency in comparison to the case of a hard and impenetrable crust like for a normal shallow surface wave. The interface mode may be excited on accreting or bursting NSs and future work on nonradial oscillations should consider this mode. Our work also implies an additional mode on large and/or cold white dwarfs with crystalline cores, which may have a frequency between the f-mode and g-modes, an otherwise empty part of the frequency domain.