Busan's (and perhaps South Korea's) most popular beach, you certainly won't be escaping the crowds if you come during the summer. From June to August this beach is packed, although the waters remain warm enough for swimming through September. The sand is rough, as it is made up of eroded rocks washed down by Chuncheon stream and shells that have been naturally ground down by the wind and sea.
Besides all the other beachside sports you can enjoy, it is one of the few places in the country where you can windsurf. Take a break from your sunbathing and have a little stroll down to Dongbaek Island. There are many islands in the South Korean Sea called Dongbaek-do (all named for the dongbaek trees that thrive on them). This one in Busan (which is no longer even an island) has a small park and a tall statue of a mermaid.