It is only fair, Itachi thinks as coughs up blood into his hand, that it all comes back to drowning in the end.
Shisui drowned in the cold, slimy waters of the Nakano. Eight minutes of his head being pushed under the water by the hands of his best friend, eight minutes of struggling, fighting, trying to survive.
Itachi drowns in his own blood. It will take him eight years to die. One year for every minute of Shisui's fight for life. Again, he is the murderer, sort of at least, as it is his own body betraying him, killing himself.
Drowning in your own blood is much more painful and slower than drowning in water, and in his weak moments he wishes he would have been able to drown in the Nakano in Shisui's place, although he knows he has no right to wish for that.
Because he knows he deserves it.
And instead of giving in, giving up, as he should, because his body is screaming at him to let go, let go, let go, dammit! he struggles on and fights and tries to survive, because he has to, because there is no other way, and because it is worth it. It has always been worth it. Sasuke has always been worth both living and dying for, and he's doing both at the same time, regardless how paradoxical this is.
"Not much longer" he thinks as he pushes himself up from the ground, ignoring the worried look Kisame is giving him, and the solace coming from this thought helps him to go on more than anything else.
Not much longer, and he can stop struggling and fighting and surviving, and finally, finally drown.