

The film's production went through much trial and error due to the crew's lack of experience of filming at sea, and both crew members and actors suffered severe seasickness during filming. įilming commenced in Seoul on with scenes of Cheon-soo and Yu-ri's first meeting, and later moved to Cebu Island in the Philippines. For her role as a jazz singer, Park Si-yeon received vocal training to improve her performance. Kim went on to film dangerous water sequences without using a stunt double, earning praise from his fellow cast members and the production crew.

Lead actor Kim Kang-woo had to overcome his fear of water, and prepared for his role by undergoing three months of intensive training, which began with learning basic swimming strokes in a pool and progressed to more advanced skills such as scuba diving. During casting he was keen to avoid stereotypes, and for the role of Kang wanted someone who looked more like a businessman than a typical gangster boss, choosing Cho Jae-hyun for that reason. Yoon said that he first began thinking about the story after watching a "very moving film noir", which inspired the type of characters he would later write into his script. Marine Boy was the debut feature film by writer and director Yoon Jong-seok.

But Yu-ri suspects Kang of killing her father, and she and Cheon-soo plot to take the drug money for themselves. His situation becomes further complicated when he falls for Yu-ri, a jazz singer under Kang's charge and the daughter of Kang's best friend. Kim is intent on capturing Kang, and faced with no other choice, Cheon-soo agrees to work as a spy on his behalf.

Knowing the danger he faces, Cheon-soo tries to make a run for it, but he is arrested at the airport by police detective Kim Gae-ko. Kang, the head of a local drugs syndicate, offers to pay off Cheon-soo's debts in return, Cheon-soo must work as a "marine boy", a mule who smuggles drugs across the open waters. However, after losing a game of poker he finds himself heavily in debt and in trouble with the loan sharks. Former national swimmer Cheon-soo dreams of travelling to Palau, and to fund his trip he works as a swimming instructor by day while gambling at night.
