Open Water (2003)

Daniel (Daniel Travis) and Susan (Blanchard Ryan) embark on a tropical vacation with their scuba-diving certifications in tow. During a group dive, the two separate themselves from the others to dive a little deeper. An incorrect head-count suggests the entire group has returned, so the boat departs. When the pair surfaces, they make out a vessel in the distance, but it does not immediately set in that they have been left behind. With sharks lurking beneath, their survival chances grow smaller.