Any more detail would entail spoilers, but I will say this: War, submarine, intrigue, ghosts/sci-fy, guilt, tragedy...all as well-done as any B+ film in its genre. I give it a solid B+...85%.
It has some really spooky scenes, and it just feels as if your drowning inside the submarine itself. A horror flick that keeps you satisfyingly scared without the use of jump-scare techniques that's all the craze nowadays. But I must say the shift from almost purely psychological tension with a dash of the creeps to full-on malediction creep-show towards the end was slightly awkward (The shift was so sharp and kind of out-there, signaled by Claire assaulting you with the notion that the ship was HAUNT---, that i almost giggled due to the abruptness of the idea if only I was not preoccupied with being scared). But if you're willing to overlook that, this movie will surely be a good watch.