Looks like I'm going to be using the setting for a one-shot adventure in a few weeks. The PCs will be crew members on a light civilian cargo sub, roughly similar to the
Ha-101, a 500 ton, 45m sub with a cargo capacity of 60 tons. It might carry a one-man autogyro for utility purposes. I've decided to go with diesel-electrics for sub powerplants, with gasoline (or less common, diesel) for aircraft. Civilian subs do not carry torpedoes but can carry deck guns; civilian aircraft can only carry "defensive" guns.
As for geography, I've decided to go with two large land masses (each about the size of Australia) about 15,000 km apart on nearly opposite sides of the world. Between them are hundreds of islands, the largest perhaps 100 x 100 km. Almost all heavy industry is on the major two land masses. Perhaps 10% of the islands have working oil wells; these are accompanied by refineries and well-defended. Many islands are poorly or completely unexplored.
(I'm taking a bit of a Traveller view to this setting, with the two major land masses representing opposing empires and the island representing low-population worlds which may be allied to one of the two major powers (those closest to them), formed their own little alliances, or independent.)