Is there a general word for both the land and water surfaces of the Earth?
'surface' works fine, Merriam-Webster explicitly includes water when it comes to the Earth's surface:
1: the exterior or upper boundary of an object or body
// on the surface of the water
// the earth's surface
You could alternatively rephrase your sentence as follows:
The airplane can land anywhere, whether land or water.
Nobody will think it can land mid-air, or on a steep mountain :)
I suggest
The aircraft is amphibious.
Lexico has
amphibious
ADJECTIVE1 Relating to, living in, or suited for both land and water.
Also in Wikipedia
An amphibious aircraft or amphibian is an aircraft that can take off and land on both land and water.
Consider changing the verb instead. An amphibious aircraft can touch down on any surface, wet or dry. You're correct that "surface of the Earth" often colloquially implies only dry land, possibly because of that word "Earth" which also means "dirt" and "ground" in so many contexts. Also, it does sound a bit paradoxical to land on water.