Known as the First, the last, and as the Seventh in the mouths of sailors, the Astral Sea is equal parts myth, legend, lies, and something more than can be expected.
The Astral Sea is a mysterious and nearly impossible to find body of water. Entrances to this sea can be found predominantly in areas of open ocean, well beyond the sight of land. On rare occassions, such entrances can be traversed by accident on rivers, lakes, underground seas, or any other body of water large enough to float a ship or raft. The challenge is that none of the locations are fixed, the Astral sea flows and ebbs with a tide not goverened by things like the moon.
The Astral Sea is often a place of thick fog during the day, as well as doldrums that can leave sailing ships dead, or nearly dead in the water. The fog has a eerie other-worldly presence to it, and is at the very least unnerving. There are many dangers to be foud in this ancient and starlit sea. Their are true leviathans here, beasts the size of mountains, and as long as rivers move through the depths. Whirlpools can burble to the surface to pull ships down to the most unimaginable of destinations.
Crossing the sea requires a steady hand and concentration. As oars splash in the water, or what meager winds push against the canvas, the helmsman must keep a steady destination. Ships that are left rudderless, or fall to despair are almost certain to never be found again, to add their slowly rotting timbers to the other hulks that have been lost to the sea, yet not taken by whirlpool or leviathan. Regardless of propulsion, be it by many oared trireme, or a hand paddled raft, crossing the Astral Sea takes six days, no matter the destination, no matter the speed of the ship.
The destination could be a port a few hundred miles away,or a mad dash for where home might have been, or the voyagers could find themselves on the shore of a strange and new world. And of course, going home is never quite as simple as just turning around and going back the way they came.
Alternate Forms of Travel
For almost as long as RPGs have been around, the Astral Plane has served as a continuum between the planes. This sort of cosmology, tied with our modern understanding of planets and physics creates a rather cold and sterile place that seems very much out of sorts with the usual grim and vigor of genre fantasy. At its very core, the Astral Sea serves the same purpose as the Astral Plane, a continuum between various planes of existance without delving into often munchkinish planar mechanics, or a discussion that leaves everyone without a degree in quantum physics either baffled or nursing a headache.