Stygia

From Fiendish Codex II: Tyrants of the Nine Hells (3.5e):

Stygia consists of a dark and frozen sea covered with ice floes. Titanic icebergs jockey for position, casting long shadows across a frosty landscape of eternal twilight.

Like a black serpent, the River Styx winds its way through Stygia's salty waters but maintains its own integrity, meandering across Stygia as it would a flattened land mass. Where it touches open ocean, its oily, putrescent waters are clearly distinguishable from the surrounding sea. The largest of the ice floes support entire cities, and the Styx cuts through these as well. Thus, the river provides comparatively easy navigation for travelers, though it can steal the identities of any who are accidentally submerged in it.

From Manual of the Planes (3e):

The fifth layer of the Nine Hells is a realm of cold and ice. Most of the plane is drowned in a sea of crushing ice floes and icebergs. The only open water is the rushing River Styx, though icebergs and fiendish sharks make navigating it quite dangerous. Lightning constantly rips through the sky, so flying creatures are particularly rare except for those that are immune to electricity.

Ice floes serve as the foundation for many devilish cities and castles.