Open Water is a very simple film about two scuba divers who are left behind at sea by their incompetent tour guides. It turns out that getting left behind is very common, and as a scuba diver who has dived with incompetent guides many times the premise is totally believable. A quick search of Google confirms how common this is. As a matter of fact the speculation in this article is exactly what happens in the film:
The film starts out slowly with a two type A thirtysomethings trying to leave their house to take their much needed vacation. You can tell from their early interactions they are one of those classic fighting couples who blame each other for their problems instead of working together to solve them.
When they come up from their dive and realize that they are far off from two boats on the horizon, one of which might be their dive boat, and they debate swimming for it. The husband points out the tides are such that there is no way they will make it and their best bet is to stay put. After all, there is no way the dive boat would leave them behind because their dive bags and dive cards are on the boat.
After hours in the water they start bickering culminating in the nervous wife's response "I wanted to go skiing!" It's a funny moment, but short-lived. Soon the sharks show up and for the rest of the film the couple faces the worst possible nightmare: being standed in the middle of the sea, at night, surrounded by sharks in a thunderstorm.
It's a truly scary moment when the lightning flashes reveal the couple surrounded by sharks flapping around in the water.
After they have both been taste-tested by the sharks it becomes clear that despite the fact that a search has been started for them by their incompetent dive guide they are done for.
There is a not a ton of gore in the film, probably due to the budget. Regardless, the lack of actually seeing the shark bites makes the film, in true Hitchcock fashion, even more terrifying.