2.4 GHz is for all models, surface or air. That's the beauty of it. Furthermore, there are no legal AIR versus SURFACE issues like there was with 72 (air only) and 75MHz (surface only).

I've heard some radio manufacturers use different air and surface protocols now. You won't be able to mix air and surface equipment but not because of anything legal/FAA--it's just different performance parameters built into it if I understand it correctly. If you're using Spektrum/JR brand equipment then only the marine receivers are compatible with the air transmitters but for the rest they should all be compatible. I've seen boaters using "air" 2.4 radios on their model yachts. I see no issue buying a 4 ch "air" radio (I prefer sticks over pistol grip controllers) and using it in a boat.