Paranormal Activity 5: Hell Underwater
Plot: The Johnson's just became millionaires from a pie eating contest, and they just bought a house under the sea. It sits in the middle of the Atlantic Ocean and what they weren't told was...mermaids have been haunting it for 34,691 years. They don't understand what they are getting themselves into, and one thing they won't be able to escape is the mighty deep blue sea.
Hell will break loose underwater when the mermaids are lose.
There's my guess (and yes, I wrote that myself).