Open Water

Open Water is a 2003 indie drama/thriller hybrid about a twentysomething couple who find themselves stranded at sea when coming back from a scuba dive and realizing their boat has left without them. Based on the true story of a married couple who were lost at sea under the same circumstances.

There is a sequel, made in 2005, called Adrift. It was not initially intended to be a sequel, but the similar premise gave them the idea that changing the title would be more marketable.

Both films have examples of:

Developing Doomed Characters: The first thirty minutes or so
Downer Ending

Tropes used in Open Water include:

Despair Event Horizon: Susan, after her husband bleeds to death and she realizes she's not gonna last much longer. Also Daniel, earlier, when he screams We paid to do this..
Did Not Do the Research: Anyone who has ever worn a wetsuit knows how incredibly buoyant they are. That's why diver wear weight belts, to counter the suit and the body's own natural buoyancy. The divers are shown ditching their belts early in their ordeal but at the end Susan doffs her tank and life vest and ducks underwater, never to resurface and presumably drowning. This is impossible to do without a great deal of effort in a wetsuit.
Driven to Suicide: Susan, by the circumstances.
Everything's Even Worse with Sharks: the sharks eat one of them by the end
Follow the Leader: Without this movie, other movies like Frozen, The Canyon and 127 Hours probably wouldn't've been made.
Hope Spot: When the boat crew realizes someone's been left behind and launches a rescue attempt. It looks like they're going to get there in time to save Susan. They don't.
Please Don't Leave Me: Susan hysterically screams at Daniel not to "leave me out here all by myself"
Say Your Prayers: A dying Daniel babbles The Lord's Prayer as he succumbs to his injuries
Shout-Out: The couple's surnames of Watkins and Kintner are those of the first and second, respectively, victims in Jaws.
Very Loosely Based on a True Story: The movie is based on the story of Tom and Eileen Lonergan, who were lost at sea after being left behind by their diving boat. As no one knows exactly what happened, the "at-sea" scenes (which comprise the majority of the movie) have been inferred.

Tropes used in Adrift include:

Gainax Ending
Karma Houdini: Dan is a Jerk Ass who grabs aquaphobic Amy and jumps with her into the water (thus kicking off the ordeal), pops Michelle's dolphin floatie, botches the swimsuit-rope attempt and ends up mortally wounding Zach trying to protect a boat he lied about owning. Naturally he survives, and only after an 11th hour suicide attempt where Amy has to face her worst fear just to rescue him.
In Name Only: Adrift was rebranded, post-production, as Open Water 2: Adrift to cash in on the original's popularity.
Mr. Fanservice: At one point the characters in Adrift remove their swimwear to fashion a crude rope. Lots of male nudity ensues.
Only Sane Man: Your Mileage May Vary but James is the only one who doesn't act like a complete idiot at some point, having gone into the water after friends. He later tries to break off a piece of the propeller to use that and the knife to get back up on the boar. See below for the outcome.
Too Dumb to Live: All the characters in Adrift, especially the idiot who grabbed the woman who was afraid of water and jumped in without putting the ladder down first. Some of the others (James, Lauren) could be excused because there were other people on the boat when they went into the water.