The Stellar Seas

When traveling between the realms, Planes, Heavens, Hells, whatever, most people use some form of Plane shift. But there is another way, a space between these worlds that can be traveled by ship: The Stellar Seas. 

Its is believe they are a part of the Astral plane, but The Seas are accessible by naturally and artificial Portals, or Ports. 

Ports

Ports come in many forms, some are great cities into themselves, some are near seamless extensions of the realm on the other side of the Portal, and still others are hidden or lost access to unknown worlds. One can set sail at from Faerun Gate, and arrive in short order at the City of Brass, the Gates of Morridin, or anywhere else they desire. The Most traveled of these Seas is the Sigil Sea, famous for its vast network of Lighthouses and its crowning jewel, the great port of Sigil.

Spread throughout the Stelar Sea, there are Portals to countless worlds. These are true openings into those plane, realms, hells, or what have you, and are the heart of all the Ports. They occur naturally, but can be created on purpose or accident. 

When a Portal opens near a population center, the culture realm often spill into the Sea. If a Portal opens near a valuable resource, the Port often becomes a strong hold of the culture that discovered it. 

This is how we ended up with the Sea as it is today, a Mix of Independent Ports, allied ports, and great empires. 

Sailing the Seas

Stellar Sea travel is very similar to sea travel, at least to some degree. Most ships, called Jammers, set sail from ports carrying things that cannot, should not, or would be impractical to travel via magic, and carry them to other ports. They navigate in 3 dimensions rather than 2, but in the most civilized areas, it can almost be routine. 

In the Sigil Sea, this is navigation is fairly easy. A Vast Network of Lighthouse create fixed points, and a skilled navigator can pinpoint Sigil and a few lighthouses, set a heading, and be on their way. The further out one gets, the more one relies on other means (Such as object attuned to a Portal, specialized Divination magic, and other proprietary means). 

Once your at the edges of the Lighthouses, you are approaching the Frontiers. All manner of Portals are scattered beyond the lights, along with all manner of folk. The closest are tend have business in the sigil space, seeking to be included in the 13th Ring, others like the navigation benefits without the oversight. The further out you go, the more you hear rumors of great treasure vaults, inhuman civilizations, undead, and more