Spelljammer 4th Edition
The astral sea is home of the gods and their servants. There are also creatures native to the realm and the various gods’ domains.
The Sea itself is not a literal sea, but sort of is. Its an infinite three dimensional plane dotted with stars, many of which are twinkling domains. The sea part comes from the fact that there is a horizon of sorts, with an astral “sea” of semi-solid fog and mist. Most creatures that are used to the natural physics of creation and the various domains typically designate this substance as a way of orienting themselves; with “Down” being the mists and “up” being the other half of the horizon, which is clear. Vessels sailing the Astral Sea typically use this horizon to locate color strands, which are navigational pointers to various domains. Astral Winds blow in the realm, allowing for practical use of sails. Fliers can fly and swimmers can swim in any direction as they could in air and water. One can even fly with a speed of 2 (clumsy) by sheer willpower. Gravity is subjective.
There are various islands and motes as well, nearly all of them have some sort of habitation. Some of these motes are destroyed domains, others are fragments of the very bodies of slain deities and primordials.