In contrast to their expressions, Sasuke's grows darker -- possibly unintentionally supporting Naruto's question at first. His posture is tense, voice forbidding as he gestures to the harbour again:

"There are plenty of crews you can join if you want. But you should know my brother is on the Murderess."

And he doesn't know if either of them have a clue just what Itachi has done for them, for the village they've fought to protect, or for the war they've all unwillingly left behind. It's unlikely, given Konohagakure's penchant for keeping secrets (and there's something that needs to change, among the myriad changes that need to happen), and while he'd explain it if the brother he has here were the same as the one he lost in his own world ...

Itachi is too young to know even of his own actions after the massacre, and he's nowhere near strong enough to deal with Naruto or Sakura if they decide to attack him. Sasuke can only protect him, and leave it to Itachi to explain as much as he wants.

"If you harm him, it won't matter what ship you get on or how far you run. You'll find the ocean is too small after all."