I have had the same problem with getting seaplanes in D-day to work. I've got it mostly working using Ares BuiltAt= feature but there is still a bug with the build queue not being able to differentiate between the two airfields.

So if you have 2 airfields (1 light, 1 heavy) with 4 pads each, you can queue up 8 light or heavy planes but it will get stuck on the 5th one because it cant build on the other airfield etc.

Of course TS/TI doesn't have this problem because aircraft aren't required to always be docked at a pad when idol. So when you get to the "5th aircraft" it will just spawn next to the airfield instead of getting stuck.

I posted a bug report about this on the Ares bug tracker but it's a low priority so I doubt it will be fixed anytime soon.

BuiltAt= does not support Aircraft. I know about this shortcoming, but I'm not too keen to touch the prerequisite or build limit systems. They're complicated, and changing a minor thing breaks five other features.