The sharks used in this film were Caribbean Reef Sharks. The cast wore chain mesh under their diving suits for protection and though none of them was bitten by the sharks, Blanchard Ryan (Susan) was nipped by a barracuda on the first day of filming. In the movie, after Susan is bitten, her boyfriend says, "It was probably a barracuda seeing what you taste like."
The school of jellyfish featured in the movie showed up on the day the crew planned to film that particular scene; it was not switched to film that scene on behalf of the jellyfish. Coincidentally, after that day, no jellyfish ever appeared on location for the whole duration of the shoot.
This movie was financed by director Chris Kentis and his wife, producer Laura Lau, both avid scuba divers. The movie cost $130,000 to make and was later bought by Lion's Gate for $2.5 million after its screening at Sundance.
This film is inspired by a true story about an American couple, Tom and Eileen Lonergan, who in 1998 went with a scuba group (Outer Edge Dive Company) to an area off the coast of the Great Barrier Reef in Australia. They were accidentally left behind due to a faulty head count taken by the dive boat crew. There were 26 other divers and 5 crew members who failed to notice that the couple was not on the boat. It was not until two days later on January 27, 1998, that the pair was found to be missing after a bag containing their passports and belongings was found in the dive boat. A massive air and sea search took place over the following three days, but failed to find them. The couple was never found.
No CG was used in this movie. Director Chris Kentis fed the sharks tuna to get real feeling of the main characters being in the ocean together with the sharks. As long as they were eating the tuna they didn't harm Blanchard Ryan and Daniel Travis.
Susan and Daniel's last names are revealed on their ID cards; hers is Watkins, his Kintner. In Jaws (1975), the skinny-dipping girl who serves as the shark's first victim is named Chrissie Watkins, and the second victim, the young boy who is killed on his inflatable raft, is named Alex Kintner.
During the bedroom scene, Susan is seen reading a magazine in which the page facing the camera has the Philadelphia Flyers logo and would seem to be about the Flyers. In real life, Blanchard Ryan's (Susan) father is CEO of the Flyers.
According to an interview with Blanchard Ryan, producers Chris Kentis and Laura Lau spent nearly half of the budget to get Stuart Cove and his shark wranglers for two days, and to make sure the two actors would be completely safe around the sharks.
In 2013 star Blanchard Ryan discussed her full nude scene in a podcast interview, including seeing it with her parents. She felt the scene was a gratuitous move but agreed it was necessary because the beginning of the movie was boring and they needed something to wake people up. When she saw it with her parents at a screening, they both had different reactions. Her father took it in stride because he comes from a sports background and has a locker room mentality about nudity. But her mom was horrified about seeing her "little girl" naked on screen. Ryan said she and her father laughed about it because her mom was more worried about Ryan taking her clothes off in front of the world than her being in danger swimming with real sharks.