It might have something to do with bad shadow settings, if you have enabled shadows. If not perhaps it is specular highlights. A directional light will have huge highlights on flat surfaces that can suddenly flash as the object moves, particularily if they are distant, or there is a low field of view on the camera. (like signalling someone by reflecting the sun with a mirror)
The ocean has a built in reflection map, so it will show up when not illuminated, but the foam will be very dark and hard to see as it is not reflective.
Duncan