Capturing territories from the sea is an important part of the game. There a number of special rules applying.
In the earlier forms of A&A (but not NGE, all retreats from amphibious invasion were prohibited. In AA2 attacking air units were allowed to retreat. In AA50 this was extended to units that entered the area by land.
In addition, bombardment can only be used in support of amphibious invasions.
From the point of view of realism, the amphibious invasion rule leaves much to be desired. Invading a hostile area, even if lightly defended, was far more difficult than moving troops across water to friendly ports.