How to build a naval alternate-history question

How can someone who is not an alternate-history writer develop a useful scenario question?

Start with a real ship cancelled by a treaty, decide which one should survive, and work backward to the change that lets it remain in service.

Begin with a real ship from the period that the Washington treaties prevented from surviving. Choose one worth preserving and ask what historical change would allow it to remain. That gives the scenario a specific point of divergence. The next step is to work out how far the surrounding world must change to make the ship's survival possible.