The first airboat were made of wood, and theres no reason why you can't, but....
Like he said, it had better be done right. Not only the considerations he mentioned above, but the engine stand, it's design, how it secures to the hull structure, how it transfers the thrust into the hull structure, and how the hull is designed to adequately except those forces.
Don't seem like a big deal, but it's not an 25 evinrude. If that falls off, well you just see some bubbles rising to the surface. If an airboat engine stand fails, you may get upclose and personal with the propeller.
It can be done, but it's gotta be done right.
Only problem with wood is, there are so many other materials out there that will do it right, at a greatly reduced wieght than wood may afford.