A cyber attack on a warship and sovereign status

Would nations treat a cyber attack on a warship as an act of war?

Probably, because a warship is sovereign territory, though a well-hardened ship should make the technical intrusion itself difficult.

A warship's status as sovereign territory gives an attack upon it serious diplomatic implications. A hardened ship should resist remote access unless an attacker can bridge separated systems. A later exchange brings the human element into that security boundary because insiders can reach spaces and equipment unavailable to an external attacker. The example of sailors severely damaging a Daring-class destroyer concerns physical sabotage rather than a demonstrated cyber intrusion, but it shows why hardening cannot be reduced to software alone. The legal question, technical access problem, and insider risk belong together.