That said, legally speaking it is true that the laws of armed conflict still apply once hostilities have began and the legality of the war itself is a separate question from whether the attack is. Under those laws an enemy warship is a legitimate target whether it has ammunition or not.

Exactly.

Obviously laws are not morals and you could still take issue on moral grounds with an action that is legal though.

I think you mean ethical? On moral grounds, you can take issue with anything, including a woman not wearing a Hijab or speaking in public. Since morals are subjective. It is just unethical to impose such morals on others.

And yes, laws are generally often misaligned with what is ethical, though I am not convinced they are in this particular case.

it is an illegal war that lacks justification under international law.

I guess, technically yes? Although that is true for pretty much any war, even arguably just ones. Regardless, that is not relevant to the topic as you pointed out.