This book, by Peter Adkison?
The TPO books are all must-have reading for deital/planar stuff. There's one chapter in TPO itself which focuses directly on the planes, while the entire Chessboards book does as well. (The other two TPO books are focused on the deity stuff, so less so on the planes themselves).
I also recommend several other planar books in a planar gaming bibliography I put together in my article "The Theory and Use of Gates in Campaign Dungeons, Part 1: Setting the Stage" in Knockspell #3 (Summer 2009); if you're interested, I'd be happy to share the list?
I am looking for more sources on the Astral outside of D&D too. Is there any one that specifies it as a sea?
In literature, the idea of an astral sea or sea between worlds has appeared before - parts of Elric's saga The Sailor on the Seas of Fate seem to fit the bill of inspiration.
If you're also looking for some good Appendix-N-style reading inspiration on the planes in general, check out Ed Greenwood's “Theory and Use of Gates” from The Dragon #37 (May 1980) and reprinted in Best of Dragon Magazine Volume 2 (November 1981 and February 1986). IIRC Roger Moore's Astral treatment in Dragon #67 (November 1982) also treats it like a sea, as the githyanki in the "Fedifensor" adventure in the same issue sail around in voidships.
The trickiest part was trying to describe what the Astral Sea is comprised of. Is it "wet"? Does it impede ranged attacks? In short, how like a large body of Earth water is it?
I would personally do something very funky with it to keep it surreal and strange, and grounded in your particular campaign world's take on planes in the multiverse. A few examples to try to make that a little more concrete:
1) IMC, the unfiltered Prime Material perception of the Astral Plane is something akin to being dead-center in a boundless sensory deprivation chamber. It's pure tedium, and an endurance test, which is why how you decide to travel through it is so important, and why most folks travel via astral cord vs. physical transfer to the plane---it may drive you mad very very quickly if you're new to the environment and get lost or are there too long.
2) What if the Astral is not physically wet, but is instead psionically wet---what could that mean, and what effects would it have, as you travel through a plane of damp psionicness?
3) What if the wet of the astral is not physical water but the mist form of the river Styx (or Lethe, or your other favorite underworld river)? If Styx, it may incite discord and hatred in those exposed to it for too long; if Lethe, it may afflict PCs with forgetfulness, etc. Perhaps the Astral is the conjoinment of all of the rivers of the underworld, and it has zones/regions/seas within it where different areas have different effects.
Anyway, hopefully something above might be useful for you
But yes, you are on the right track with the link above.
