The twist ending was actually really good and well done!
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i thought the ending where it shows both sisters making it to the surface, then being attacked by a shark before being pulled out only to turn out that Mandy's character was still on the bottom of the ocean (with Claire's character actually having been killed by the shark attack)...i thought that was executed really well. when Lisa (Mandy) is on the deck of the boat and sees the blood from her hand floating i just figured she was saved but hallucinating. the fact she was still down below and that her sister Kate really had been killed/eaten was a well done twist. having it turn out that Kate really had been killed (presumably) by the earlier shark attack was a really sad/haunting ending.