A good horror film will have you unable to sleep that night. A great horror film will have you thinking about it for weeks on end. There's only one film that comes to mind whenever you go in the ocean.
Despite all that, it is classed as a thriller, although it is pretty horrifying; I first saw it at the age of three, and even now 20 years later, I will not step foot in the ocean (I also think watching Titanic at 4 contributed to that, haha.)
On a side note, my little sister watched it when she was eight; she enjoyed it, although when Quint got eaten, she was so scared that she cried

hno: