Based on a true story, however a lot of the film is speculation.
Daniel and Susan are a busy working couple but manage to find some time in their busy schedules to take a scuba diving holiday, on the 2nd day of their vacation they go on the scuba diving expedition and during the safety talk a man on the trip informs the people running the tour he has forgot his equipment and can not go, they all go into the water and start exploring , half way through a young couple come back up and they adjust the count to say they have come back on the boat however the gentleman who forgot his gear borrows the young ladies and the other man goes back out with him not adjusting the count to say they have gone into the water, everyone gets back unto the boat and thinking they have everyone back go back to the resort, Daniel and Susan emerge to find the boat has gone.
They start to think that they will obviously realise that they have left them behind and return, after a while they also notice they have drifted from the original site and Susan starts to feel unwell as she she has been drinking sea water Daniel tells her to lay back and relax and she falls asleep, she awakes alone and starts shouting for Daniel he hears her and they start swimming back to together but on the way gets bitten by either a barracuda or a shark, they then realise that sharks have been beneath them for a while, they see a buoy and figure to start swimming towards it then Daniel gets bitten by a shark and starts bleeding heavily Susan tries to stem the bleeding but it carries on bleeding and Daniel even with Susan trying to calm him goes into shock, night falls and during the night Daniel dies.
The crew go back onto the boat in the morning and find Susan and Daniel's personal effects and realise that the count they made must have been wrong and search and rescue is mobilized.
I am ending it here even though there is only 5 minutes of the film left but as you can probably guess there is going to be 2 possible endings and guess what, I am not going to tell you so ner ner !!, but on the whole a very enjoyable film and to say it was filmed on a shoe string budget they did well.