What I was thinking about was just to add a "seaplanes can land in this airport" checkbox. Marseille, Montpellier, to name cities next to my home, are airports next to lakes, and seaplane operations are obviously possible from there. And an airplane could have "seaplane landing" checkbox as well - and maybe even a "can't land on ground" checkbox(with a 0m landing requirement, or a 9999m, or whatever makes the job according to the database structure I do not know).
Then you add seaplanes, with or without ground landing capabilities, and players do adjust.