Creature from the Black Lagoon
I'm surprised so many people here talk about aliens. I didn't see any signs of aliens in this movie. This was much more likely a "creature from the black lagoon" type of situation (as in the classic horror movie of the same name from 1954). Some mysterious frog-like creature(s) lived in the lake, the local people knew about it, the girl probably also knew about it because she had grown up there, and as the girl came back she fell under the spell of the creature. I would call it an intelligent, though very loose, reworking of the 1954 movie.
That's my interpretation - in addition to the artsy-hipster-psych interpretation of the movie being an allegory of the horrors of marriage.