What might cause these "wave clouds" near the horizon? Could it be gravity waves?
Identification of the phenomenon based solely on the photograph is unfortunately inconclusive, although several reasonable possibilities exist. While a wavelike structure is apparent, exact identification of the phenomenon is difficult due to the lack of cloud condensate (which would better illustrate the local vertical air motions ultimately driving cloud development) and of corresponding local weather observations (which could suggest which phenomenon is most likely to occur). Three possibilities seem most likely:



Horizontal Convective Rolls - a mode of convection in which clouds (and corresponding upward vertical air motion) preferentially organize in approximately linear lines, with downward vertical motion between (image from linked source). These clouds are observed over both land and water surfaces, although formation over water is more likely in cold weather when the water is relatively warm compared to land. This mode is possible, though perhaps least likely, given that weak clouds are visible across the horizontal length of the photograph - well-organized horizontal rolls commonly have clear air between the lines of cloud.  (image source)





Gravity wave clouds - clouds that form in the upward motion regions of large-scale "waves" of vertical motion propagating through the atmosphere. Gravity waves are a common phenomenon occurring in many different atmospheric environments, with the resulting clouds varying greatly in appearance depending on the strength of the vertical motion and the amount of atmospheric moisture available for condensation. Given this, gravity wave clouds are a reasonable possibility based on the photograph. (image source)





Kelvin-Helmholtz wave clouds - An overturning motion that may develop at the interface between two atmospheric layers with substantially differing horizontal winds, with cloud condensation (assuming sufficient moisture) taking on an appearance similar to rolling ocean waves (image from linked source). This explanation also seems reasonable, given that air masses with distinct characteristics commonly occur in the lower atmospheric boundary layer and between ocean and land masses. Anecdotally, the clouds in the photograph look most visually similar to near-surface Kelvin-Helmholtz wave clouds I've seen, although again, the lack of actual cloud condensate makes this difficult to assess with any certainty.  (image source)