He's 2/3 submerged below the surface where they surf

from Lupe Fiasco – Lupe The Killer Lyrics on Rap Genius

Meaning

He is playing off the line before where he says he is “immersed (submerged) into his murk (murder, or metaphorically his rhymes).”

So he is not only very deep into his art but also his art in comparison to an ice berg is also very deep. And the surface being average Im guessing or mainstream.

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