Open Water is the true story of a couple that went scuba diving from a dive boat and were never seen again after accidentally being left out in the ocean by a dive boat crew.
Daniel Kintner (Daniel Travis) and Susan Watkins (Blanchard Ryan) had crazy, hectic lives that didn't allow them to spend enough time together.
**Be forewarned, there is a gratuitous nude shot of Susan early on in the movie.
They decide to get away to the Bahamas for a scuba trip to relax and try to get in touch with each other again.
On the second day of their vacation, they decide to take a trip on a scuba diving boat. When everyone is on board, the diving instructor marks "20" on his note pad to record the head count. On the trip out, someone makes a comment about sharks in the water; but the dive instructor laughingly dismisses it.
When they reach the dive spot, divers go into the water as couples, using the buddy system. The dive is set for 35 minutes; and divers are instructed to be back at the boat within that time period.
A short while later a couple comes back on board. The woman had a problem with pressure equalization and had to abort the dive. The dive master marks three "ticks" on his sheet to record three people on board. Seth, who had forgotten his dive mask, persuades the couple to lend him one; and then persuades the man of the twosome to join him on the dive. The dive master does not pay attention to the couple leaving the boat and does not correct the head count on his note pad.
Meanwhile, Daniel and Susan decide to separate from the group and explore a different area underwater. As the 35-minute time period approaches, couples begin to return to the dive boat; and the diving instructor ticks off their numbers. Prior to leaving, he shows 20 on his pad, when the actual number should be 18. The dive crew believes that everyone is on board and leaves the dive spot.
What actually happens to Daniel and Susan after this point is pure speculation.
The next morning their belongings are eventually discovered on board the dive boat. The IDs are checked; and the dive instructor remembers seeing them on board. Their hotel rooms are checked; and the beds have not been slept in. At 8:55 AM helicopters, planes and boats are sent out on a rescue mission.