Hm! OK, that means that you've got very limited ways to score. In :RA, you can just fall back to Plan B, but in :IO, it's hard to even turn the ships around.
Most of the time, you keep HVAs close because someone can just drop a company on them at any point. This means the best thing to do is put them far away, removing them as much as possible from the tactical space.
I think you'll have to fall back to source material here:
In WWII, the job of the navy was largely to support/prevent troop movements. Nation 1 would need to move soldiers, materiel, or supplies between islands A and B, so they'd send them with an escort. It's the job of Navy 2 to stop them.
So what you'd want here is a wholly different setup for determining score, and likely conditions that included getting an HVA from point A to B.
Fortunately, I have plans for this kind of thing! But it takes actual game design to make it work!