A common misconception by laymen is that the force is derived from "pushing against" the water/air as in pushing against a wall. In reality, the force is derived from the acceleration of the water/air. The force would be the same in a vacuum for the same amount of water/air accelerated even though there would be no air/water to push against.
It's not a misconception at all. Pushing against a brick wall (or against the ground) is the same thing as pushing against a mass of air or pushing against a jet of hydrazine in a hard vacuum. In each of these cases, it's Newton's Third Law that moves you about. In the case of a brick wall or a planet, the reaction mass is gigantic (never infinite, however), whereas in the case of hydrazine molecules, the reaction mass is tiny. The principle is the same, though. The differences are only quantitative.