Surface waves are an important mechanism for the redistribution of sediment on shallow marine shelves, and are commonly interpreted as comprising two distinct populations: fair-weather waves and storm waves, the latter of which are generally thought to penetrate to greater water depths. Here we used >2.3 × 106 spectral density estimates for the surface ocean collected between 1996 and 2008 from 32 buoys in the Caribbean, the Gulf of Mexico, and the western Atlantic to test the hypothesis that surface waves in the modern ocean comprise two size modes. Although distinct wave size classes occur in some individual measurements and over the time scales of some individual storms, time-averaged frequency distributions of wave size are unimodal. Thus, there is no empirical basis for presupposing a distinct bimodal separation in the size of fair-weather and storm waves, or in the manifestation of such differences in stratigraphic successions. Instead, there is a continuously increasing probability that a wave will reach the bottom with decreasing water depth and a separate probability that describes the hydrodynamic state of the sediment-water interface. Wave size does, however, exhibit significant geographic bimodality. Locations in the relatively protected Gulf of Mexico and Caribbean regions have modal wavelengths that are ∼50 m less than waves at locations along the western Atlantic. Time-integrated estimates of the depth of wave penetration provide empirical constraints on the paleo-water depths of ancient sedimentary deposits and highlight differences between sheltered shelf environments, such as those that characterized many ancient epeiric seas, and open-ocean–facing, narrow continental shelves.