It’s, by definition, not a war crime. Military brass bands are eligible targets as are unarmed naval ships.

The U.S. definition balances military necessity against humanity, which in this case is not looking good: no mitigating attempts to reduce the death toll – no warnings, no attempt to disable the ship – and since the ship was trying to get permission to dock in Sri Lanka or India at the time, it’s hard to justify a claim of military necessity for a ship which was either unarmed or very lightly armed and clearly posed absolutely no threat to the much larger and better equipped U.S. navy. It’s unlikely to ever see a formal trial but I think quite a few people will see it as if not an outright war-crime, at least a betrayal of military honor.