Disclaimer: I don't own Naruto. Also, I have quoted the poem 'Not Waving But Drowning' by Stevie Smith, without permission but with much respect. It stuck in my head the first time I read it, back when I was little, and it only recently occurred to me how well it fit Itachi and Shisui. I have included the full poem as a separate chapter for those of you who are interested.
Itachi remembers the look in Shisui's eyes: not betrayal, but bewilderment. Not anger, but confusion. Not blame, but then again Itachi has enough blame for both of them.
He remembers, and suddenly Shisui is there: drowning, drowning, drowning.
Nobody heard him, the dead man,
but still he lay moaning:
I was much further out than you thought
and not waving but drowning.
Itachi remembers the look in Shisui's eyes, and he drowns, drowns, drowns.
Sometimes in the summer they swam in that river, and Shisui would duck under and splash and squawk and every time Itachi fell for it (fell for it over and over and over, or only pretending to fall.)
He would go to help, and every time Shisui would laugh and tease him and call him a mother hen- except once; when he didn't, couldn't, never ever would.
(And not waving but-)
Itachi remembers how he held him under until he didn't need to anymore, remembers the way Shisui went limp in his grip. It felt so strange, so sudden. So pointless.
Oh, no no no, it was too cold always
(Still the dead one lay moaning)
I was much too far out all my life
Shisui dies.
Itachi drowns.
And not waving but drowning.