If the astral plane is a "sea," you are a fish
There is a description of the astral plane in the Dungeon Masters Guide (p. 46-47, bold added). There, it states:
The Astral Plane is the realm of thought and dream, where visitors travel as disembodied souls to reach the Outer Planes. It is a great silvery sea, the same above and below, with swirling wisps of white and gray streaking among motes of light like distant stars.
Most of the Astral Sea is a vast, empty expanse. Visitors occasionally stumble upon the petrified corpse of a dead god or other chunks of rock drifting forever in the silvery void. Much more commonplace are color pools- magical pools of colored light that flicker like radiant, spinning coins.
When you hear that the astral plane is like a "sea," it's natural to picture it from the surface of the water, because that's the main way that humans interact with the sea. But the astral plane is more like a sea with no surface or floor. Although you might find solid material there, the concept of an "island" is somewhat misleading. Also, note one other important detail (DMG, p. 47):
Creatures on the Astral Plane don't age or suffer from hunger or thirst.
So you could find any number of living things in the astral plane without finding any "land" for them to live on. Unlike the material plane, traditional life isn't necessarily tied to any particular ecosystem. In other words, the plane could essentially contain anything or nothing. The only limits are what you can think of: because thoughts are precisely the source and nature of the astral plane.