A beach bum is someone who spends a lot of time hanging out at the shore, swimming in the ocean or lying on a towel on the sand.
If the beach is your favorite place in the world, you could get a job as a lifeguard. But if you love the beach and don't need to work, you can just be a beach bum, someone who simply loafs around there. One of the first printed examples of this term was a reference to California surfers in a 1950 issue of Life Magazine. Bum is an American English word meaning "tramp or loafer," from a German source.