Or the astral sea could be a 2-dimensional sea. And planes float on the sea's surface, but cause the whole sea to sag down around them. As an astral ship approaches a plane, it gets pulled down by the current of the sagging astral sea into the spherical plane. Kind of like a 2-dimensional version of actual space.
Would be kind of cool, but I'm sure it's more like normal space. Hint: There's a picture of the Astral plane on one of the book covers. You can tell it's the astral plane by the monster...