[Mass Combat] PLAUSIBLE army diversity in settings

[downer]

Well, looking at it, the navy of those island people would probably be composed of a lot of small units - corvettes, armed trawlers, submarines. They use these to keep their waterways under control and keep their waters clear of commerce raiders. If anything more powerful - battleships, carrier groups - shows up, they call in the air force, which is based on the islands, with enough range to strike the invader.

That still means that their long-distance trade is open to whatever their enemies want to do to it. Carrying on international trade will be next to impossible with this configuration, if major surface units go after your convois. Submarines might still be kept in check by destroyer and corvette escorts. They might plan for this contingency by holding a unit or several of higly specialized planes in reserve, which are designed to attack such surface units. Basically, large fuel supply, powerful anti-ship weapons, stealth capacity. So when the enemy declares war, they send out these units to sink whatever large commerce raiders they can field, then use their smaller units to keep their convois safe from small-scale commerce raiding by subs and auxiliary cruisers.

It might just work. After all, TL 8 carrier groups are not primarily a weapon of sea warfare, they are a way of projecting air superiority. If that is not what you want, you wouldn't need them. The specialized planes outlined above would be sufficient to sweep the seas clean of battleships and carriers, and might serve as a deterrent to enemy naval deployment, and convoi warfare is a matter of lighter units anyway.