I think i have heard this song a thousand times, if it isn't more. In a way I think it is very creepy, especially if you know that the instrumental part symbolises the time it takes to lose conscience under water (that's what is heard anyway).
There are a lot of theories about this song. I myself don't have a very clear one, but what I suspect is that it's about someone who commits suicide by drowning herself. But then again, I think that it's very obvious. I've looked something up and discovered some nice theories.
This theory (the second last post) says that it's about Ophelia from Shakespeare's Hamlet. The prove that has been given makes it a very strong theory, except that someone asked Emilie if it was about Hamlet, and she said it was not. I have no idea since I haven't read Hamlet yet, but it's on my to read list with a lot of other books.
Another theory is about 306 deaths in the Seine.
I have no clue which theory is the right one, and I'm sure their are many other plausible ones, but I like it that we don't exactly know what it's about. The mystery gives another dimension to the song.