This was on HBO last night (I originally saw it in theaters).
Plot: Husband and wife scuba divers get left behind in the ocean by incompetent scuba-diving-boat owners.
Overall: I liked it a lot, not sure you will.
Tension Level: Plenty
Mostly See: Lots of Water and Fins of Sharks
Gratuitous Nudity: One ridiculous scene in the beginning to keep the male audience hanging around for more.
Based on a True Story: Yeah, kinda. The lead actors have said they have "heard about such an incident happening" but couldn't name any incident specifically.