You take islands just like in the original games, the japanese have transports and destroyers that are in range to do so. You don’t get IPCs or VPs for ‘capturing’ a convoy. Convoys are a bit confusing. If you own both the land/island AND the convoy sea zone you get IPCs and/or VPs. If you moved any naval unit except a transport through (blitz) or into a sea zone that contains a convoy you either 1) put a flag on the convoy symbol if it doesn’t belong to you or an ally or 2) ensure there is no enemy flag on the symbol if it belongs to you originally.
So, it’s possible to capture an island using only transports and land units and not get anything for it because you have not ‘captured’ the sea zone convoy route. Seems odd that some islands you can caputure and get an IPC without requiring a combat ship to also capture the sea zone and yet other islands and mainland territories you must. The rules state you can’t capture sea zones but it’s not exactly correct is it….