Killer Mermaid (2014) on Netflix

Two young American women go on a Mediterranean vacation and uncover the watery lair of a killer mermaid hidden beneath an abandoned military fortress. What was once a carefree adventure becomes a deadly fight for survival.

Two American girls travel to Montenegro on vacation to visit an old friend. While they're there they decide to venture to Mamula, an abandoned military fortress located on a remote island. During their exploration of the fortress they discover that they are not alone. Someone else is on the island with them and he will stop at nothing to protect it's secrets. There is a darkness hidden beneath the island and the terror has just begun.

—Srdjan Mitrovic