It's been a long time since I read this and I just remembered about it again, so forgive me if there's not much that I recall.

I think it was a Seinen. I remember there's this high school girl who's suddenly threatened by some supernatural incident (I forgot what it was) and was saved by the MC who's her classmate I think. The MC has some kinda powers that's related to a pocket watch? (I think), but I really remember that he had a long black-haired spiritual/ghost companion girl that looked like Hagoromo Gitsune from Nurarihyon no Mago. She seems to have been sealed/bound(?) to a house/room and haunted it for a long time, until the MC came and she decided to be with him. I think he's part of some supernatural organization that solved such incidents that threatened the girl's life and there's some other people in that organization who wanted his ghost companion.

I remember in some further chapters, they were on a beach and there's this huge shark about to eat the girl.

That's it I think. There were only a few chapters back then (probably not even a dozen).

[img]http://i.imgur.com/OWQqJVH.png[/img]

Probably not what you're looking for, but give it a try: Dusk Maiden of Amnesia (Tasogare Otome × Amnesia). I haven't read the manga, only seen the anime (which I don't remember very well 😀), so your description doesn't feel familiar, but the girls look similar and the genre is supernatural. Also, if I remember correctly, she haunted the school for a long time and after the MC came, she decided to be with him. Well, I don't know, but hopefully it's what you're looking for. 🙂

I actually already read that one, haha. Big part of the reason why I was interested in this manga is because the ghost had similar designs to Hagoromo Gitsune from Nurarihyon, Yuuko from Tasogare, and Rea from Sanka Rea. I was rereading some Tasogare manga and suddenly remember about this one, but can't seem to find it even after searching through tags and genres. The only other thing I can recall is it has a relatively "clean" art style?

[img]http://i.imgur.com/OWQqJVH.png[/img]

Hmm, sorry, I don't think I'll be able to help then. Hopefully someone else has read what you're looking for and will be able to point you in the right direction 🙂 Good luck! 🙂