[8FS_Bulau]

Yes, test runway 4 with a homebase on it makes you spwan in the air,
but I was wondering how you can make waterplanes take off from the water in dofight maps.

Yes, some maps have water aerodromes. Finnish Gulf has some, NTL_Darwin, maybe Norway...can't recall.

Regarding the OP and airspawns, Test Runway 4 gives airsspawns if placed on water, or uneven land, but if placed on level ground, you will spawn on the ground.
Another way to get airspawns is with the latest Zuti DSMods, which adds a checkbox for airspawn to the Home Base in Object Viewer.