Waters Above (2017)

Rules for Adventuring in the Astral Plane

From the introduction:

The Astral Plane: a vast, silvery sea where thought takes form and dreams dwell. Divine planes hang suspended in its waters like planets or islands, and the whirling currents bear a precious sprinkling of habitable land to succor the dead in their unfortunate afterlife. Heroes and villains from every age mix and mingle here, where fish forage alongside deer and sharks contend for their kills with wolves, and one is just as likely to find a forest of kelp as a one of pine. Githyanki pirates sail the surge of its psychic tides and adventure swims in its depths, ripe for the right group of heroes to rectify what the gods have neglected.

The astral plane has long been described as an ocean, but this book expands on the concept, imagining the physical, mental, biological, and social consequences of life in an ocean of thought and dream. Within this book, you will find the astral sea described like a real ocean, according to depth, with unique creatures, locales, resources, and factions in each layer. You will also find all the rules you need to run adventures or campaigns in this reimagined astral plane, including everything necessary to allow your players to build, customize, and sail their own astral vessel.