4 adolescents sneak into a public pool (of some sort). This automatically tells me that whatever they are doing, they are not supposed to do. The first couple dives into the water and they are the only two kissing and touching and all that stuff. It's not until the brunette gets uncomfortable and dives into the water too that the water starts to symbolize something.
To me, the water symbolizes sexual urge, and since the couple were the first to dive into the water, and sit in the water afterwards, their lust was strong. Meanwhile, you have the conservative brunette who not only is too shy to kiss the boy but tries to escape him by going into the water. Meaning that she doesn't know what she's getting herself into, until she sees the worm thing crawling around in her bikini/panties/whatever.
THIS shows that she has sexual urge but, doesn't want it to take over her or whatever it was going to do to her, like what happens to the couple shortly after she gets out of the pool. Now, the boy that wanted to kiss the brunette never really dove into the water himself, which i think means that he has a better sense of what he feels is love rather than lust (?)
Anyway, what happens is the first couple is obviously parasite monster things already; their lust has eaten away at them and has turned them into monsters. So the girl, still being her conservative self is able to escape these things, meanwhile the boy is captured by the couple. Now the pool room itself is a tomb of lust that she cant escape and the monsters will soon overwhelm her. Her decision is to dive straight into the water.
This means she decides to accept whatever this lust is and what it'll do. Now this is where it REALLY get's complicated, but i was able to think of what the ending might possibly be; when she touches the other side of the pool and emerges in that other world, she sees this shadowed being and almost instantly her eyes explode. What i think this means, is that in letting herself be trapped in this lustful situation in which she cant escape forcing her to embrace her lustful feelings, she has entered a world in which her young mind cannot fully understand/comprehend.

I dont know if this is what this video means. I could be wayyyyy off. But this is my interpretation.