The sound engineer for the Beach boys was one of the first to develop the plate reverb sound, where he used a metal plate on the opposite side of the microphone and the sound waves would hit the microphone twice, once on their way by and once after bouncing, or reverberating, off of the steel plate. This is why we associate reverberated guitar with beach music, because beach music started with that sound.
Why do we associate reverby electric guitar with the ocean?
c.f. surf music and spongebob's sound cues...