The ocean beyond the third continent, 32,00,000 yojana wide. Ordinary water.
The first ring that lies wholly outside the human world. Nothing here is reachable, and the scriptures accordingly say very little: a width, a name, and the fact that the water is water.
That silence is worth noticing rather than filling. The cosmology is enormously detailed about the two and a half continents where liberation is possible and goes quiet almost immediately outside them. The detail follows the doctrine, not the geography.
The middle world, ring by ring - Puṣkaroda highlighted