“Undertow (demo version)” is the 7th track released by Passion Pit from Tremendous Sea Of Love and the 5th track on the album. It was posted on YouTube on February 22, 2017 (it is now deleted) by the Wishart Group, an organization founded by Michael Angelakos (Passion Pit).
It’s only the second of the first 7 songs that was somewhat teased before it was released, Angelakos having tweeted a portion of the lyrics before the song was posted.

It is only a demo, so there are parts of the song that are unfinished/unpolished; however this version of the track is the one that appears on the album.

The word undertow has two definitions: one is rip tide, a current in the ocean that pulls swimmers out to sea and away from the shore, and the other is an underlying theme, emotion, or influence in something like a novel that leaves an impression on the viewer/reader. The word having two meanings adds an additional quality to the meaning of the lyrics.