You know how sometimes a movie seems like just an ordinary slasher flick, and then it gets weirder and weirder, and finally you get to a scene where a mortally wounded woman is climbing over a pile of her dead duplicates from another timeline, which are being eaten by seagulls? Yeah. That.
The above clip is when one movie, in particular, reaches a pinnacle of maximum weirdness and intensity. Spoilers ahead...
The clip actually comes from 2009's Triangle, which I'd actually thought was just a standard "trapped on a boat with a killer" movie, with a bit of weirdness here and there. The movie starts out that way, and then turns into a trippy film about a time-loop in which a woman keeps running into her slasher self from another version of the loop. (That's Jess, watching her alternate self kill another alternate self in the clip above.)
I became curious about Triangle when the Straight Dope Message Board described it as "Primer on a boat." But it's actually more like "Timecrimes on a boat." It also has more seagull-related creepiness than any "deadly boat" movie ought to have. It's a bit cheesy in parts, but well worth checking out.