I haven't found any conclusive evidence, but I'd be willing to bet, that this film wasn't made as a sequel to the indie-shark film: Open Water - That the marketers saw a film with a similar idea, and thought that the film would get more exposure on video if it had a familiar name attached. See the poster and the teaser trailer as evidence, neither mention Open Water 2. Plus another huge thing missing from this film, that I can't believe they would do if they went into production as a sequel: THERE ARE NO SHARKS!
They hint at it at least two times; one of the girl's feels something bump against her leg, and then somebody gets badly cut with a knife and we're shown a large trail of blood, alas no sharks appear. Don't let that dissuade you from this well made little thriller.
In Adrift we meet a group of young folks who are out for a day of fun on their friend's yacht. There are three couples; Amy and James, who've just had a baby, Amy suffers from a major case of water-phobia, the reasoning is revealed throughout the film; there is Zach and Lauren who seem like a nice normal couple; and then there is Dan and Michelle, it's Dan's boat, he's a wealthy-playboy who just met Michelle a few days before; throwing the rest of the group off, since they've all known each other since high school. They set off on Dan's yacht into the Gulf of Mexico.
After they've passed well beyond being able to see any land, they decide to stop for a little dip in the ocean. Everybody but Dan and Amy frolic about in the water for a bit, Amy is content to stay on the boat with her baby due to her aforementioned phobia, until Dan, being a total jackass decides to grab her and jump in. She instantly goes into shock, ruining everybody's good mood, which is about to get much worse in a couple minutes when they realize that nobody bothered to put the yacht's ladder down, leaving them stranded in the water.
What follows next is pretty terrifying and fairly realistic. Every time I thought of a way for them to get back on the boat, the characters tried it; including standing on each other's shoulders, using their clothes as a rope and finding some sort of hatch. There was only one scene were a character does something really stupid, and it's sort of forgiven by a later explanation. So for the most part, you can believe them when it's said that this film is based on actual events. People do stupid things all the time, get themselves into bad situations, it's how you handle yourself afterwards that sets you apart. Most of the characters are dispatched in tragic, yet believable ways.
While the film is not a classic by any means, it's a fairly tight little thriller. The mostly unknown actors do a good job of conveying the terror that they're facing. The camera-work is inventive and the editing is good, keeping a good quick pace. I could have done without the repeated flashbacks to Amy's reason for hating the water, they stretch it out way too long, when it's fairly obvious what happens the first time around; but it's all shot in super-8, my favorite format, so I could forgive it. If you go into it not expecting a shark frenzy, are able to put yourself in the character's swimsuits, you might have a good time with Adrift.