Sea Clouds

Sea Clouds (海雲, Umigumo) is a special kind of clouds. They are similar to water, which means that you can sail on it if you´re not to heavy to sink through it, or travel through using Waver or Jet Ski. But they are lighter than water, and as such, things that drop into it sink faster than in normal water. However, they are still considered to be a ocean, and as such, can affect Devil Fruit users like a regular body of water would.

According to Pagaya, an element contained in the Kairoseki called Pyrobloin form sea clouds (as well as island cloud, depending on the density of the water) when it absorbs water in the sky.