Siren
(2010)

Rachel (Anna Skellern), her current boyfriend Ken (Eoin Macken), and her past boyfriend Marco (Anthony Jabre), go out boating for the weekend and get a stalled boat. They wind up finding a sailor who dies on the boat and lead them to a stalled island, meeting the mysterious Silka (Tereza Srbová), (the 'Siren' of the film).

The movie then stalls for an hour, but all you need to know is that everyone has sex with Rachel (including Silka, who wants her for herself and wants the guys dead), and  the guys, after realizing this, want off the island...they won't make it.

Silka kills both Marco and Ken with her song, and then gets stabbed in the throat (and dies in the water) by Rachel. Rachel flags down someone in a motorboat, though the film is ambiguous as to whether she wants off the island or wants to turn into the new siren (probably the later).

Thanks Haterade!