Open Water
Open Water is a low budget movie (approx US$136,000) that was filmed almost entirely on relatively cheap digital camera technology.
The acting was rather poor (kept expecting the funky porn music to kick in and for the characters to lose their kit), but by the time they'd been deserted in the water it was clear that the characters were secondary to the situation.
Relatively short, but it stayed away from a corny ending, which was welcome.