Fathom's theme song is Safe Harbour (Black Flag) by Somastate

Lyrics:

"Fathom" (the second word in the song is literally 'Fathom')

"The waves will crash" (he is a SeaWing and lives in the ocean)

"Fearful night" (when Albatross snaps and goes crazy)

"For there is danger here upon the deep" (Albatross again)

"Don't you fall asleep" (he is afraid that Indigo will fall asleep forever, aka die)

"She haunts us here with hungry cries" (let's say that the 'she' is 'he', it could be talking about Albatrosses ghost)

"I'll never long for safe harbour here" (it could be talking about how he never longs for safety when he's with Indigo because he's safe then, or it could be about how he never longs for safe harbour in the ocean or with Darkstalker because he knows it'll never be safe)

"I was born to feel the storm" (he was destined to almost lose Indigo and to help Darkstalker, both of which were difficult, like flying in a storm)

"We can never hold the wind within our hands" (you can never stop Darkstalker, a force of nature and often destruction, from becoming evil)