The story behind this film is sound: A town is submerged to keep a fiendish demon from taking over the world. Two kids unwittingly release this evil & 40 years later it's ready to rampage over the earth. Unfortunately, the story is thrown off kilter by awful acting and characters you just can't care about.
You have the mother/reporter who looks like she's only a couple of years older than her daughter & whose overt indifference to her daughter makes it impossible to care what happens to this character.
The character of the daughter is not quite so irksome but ruined by awful acting on the part of actress Charlotte Salt.
Mordecai Salas, the demon-type character, is wonderfully creepy but feels slightly like a ripoff of the Reverend Kane character from the Poltergeist trilogy.
The creature/makeup effects are excellent - the non-supernatural effects are dreadful. The green screen technology is obvious, painfully so during the diving scenes and the divers appear as large as the roof of a house. Forced perspective can work fine--just look at Lord of the Rings, but it's very sloppy in this film.
Watch the trailer and that should suffice.