The Tool FAQ offers this explanation of the lyrics:
Undertow is a song about repeated drug use. The voice that he hears is some sort of craving for this drug (or something), it suggests nice things to him, and beckons him. When it surrounds and drowns and wipes him away, he is obviously succumbing...
[Verse 1]
Two times in
I've been struck dumb by a voice that
Speaks from deep beneath peerless water that's
Twice as clear as heaven
Twice as loud as reason
Deep and rich like silt on a riverbed
Just as never ending
Current's mouth below me
Opens up around me
Suggests and beckons all while swallowing
Surrounds and drowns and wipes me away
But I'm so comfortable, so comfortable
[Chorus]
Shut up, shut up, shut up, shut up
You're saturating me
How could I let this bring me
Back to my knees?
[Verse 2]
Third time in
I've been baptized by your voice that
Screams from deep beneath the cold black water that's
Half as high as heaven
Half as clear as reason
Cold and black like silt on the riverbed
Just as never ending
Current's mouth below me
Opens up around me
Suggests and beckons all while swallowing
Surrounds and drowns and wipes me away
But I'm so comfortable, so comfortable
[Chorus]
Shut up, shut up, shut up, shut up
You're saturating me
How could I let this happen?
Why don't you kill me?
I am weak and numb and insignificant
How could I let this bring me
Back to my knees?
[Bridge]
Euphoria
Euphoria
Euphoria
Euphoria
[Outro]
I'm back down, I'm in the undertow
I'm helpless and I'm awake, I'm in the undertow
I'll die beneath the undertow
There doesn't seem no other way out of the undertow
Euphoria
Undertow was written by Adam Jones & Maynard James Keenan & Paul D’Amour & Danny Carey.
Undertow was produced by Tool & Sylvia Massy.