Not Waving But Drowning is an instrumental track on Sparkles* Soundcloud, released on the 23rd of January, 2014. According to the description, Sparkles* has never played the song to anyone, until he found it. He states "it's quite short and watery".
It starts with water-based sounds and a piano, with the water fading out to be replaced with a violin, while the piano continues. This goes on for about 1 minute 13 seconds (The track itself being 1:23), when an electronic fade-in kicks in (which abruptly ends, maybe suggesting it leads into another track). This track may have something to do with the " ATLITS - Complete" video, as in the the beginning you can hear bubbles, and as with said video, there are water tanks present which bubble throughout the video.
Not Waving but Drowning is also a poem created by Stevie Smith, a female British poet, in 1957. The poem is as follows;
Nobody heard him, the dead man,
But still he lay moaning:
I was much further out than you thought
And not waving but drowning.
Poor chap, he always loved larking
And now he’s dead
It must have been too cold for him his heart gave way,
They said.
Oh, no no no, it was too cold always
(Still the dead one lay moaning)
I was much too far out all my life
And not waving but drowning.
There are a few interpretations of this poem, however most are too vague. One states that the drowning man is Smith herself, which could mean that taken into the Digital Haunt, Cassandra is the one drowning. It could be a metaphor for crying out for or wanting help and/or attention (hence the drowning) but appearing friendly and mentally stable, et cetera. Other interpretations (of the poem) can be found on the poem's Wikipedia page, here.