A sea of its own for every world that has one: waves raised by the weather's wind, swell, white water where they break, surf on the beach, seaweed in the shallows, and a camera that can go half under.
A first sea, not a finished one (0.1). It reads as a sea at a glance; it does not yet stand beside a photograph of one.

For Kerbal Space Program 1.12.x. Needs Keystone, a separate download (on SpaceDock): this mod does not load without it. With Natural Weather, which is not needed, the sea goes by the weather's wind; without it, by a light breeze.
Copy the NaturalOcean folder from the download into your KSP's GameData, beside Keystone.
In the Keystone window (the keystone button on the game's toolbar, or Alt-K; Option-K on a Mac): how rough, how clear the water is, whether craft ride the waves, seaweed, and whether other seas are put out of sight.
Windows 11 (Direct3D 11 and OpenGL) and Mac: tried. Linux: never run.
More: the long description (everything in full, and what was tried on each system) and the rest of the wiki.
CC BY 4.0: use it, change it and pass it on as you like, as long as you credit Ishiaki and link back here.
Raw stats are from the beginning of time until now. Each follower and download entry represents one hour of data. Uneventful hours are omitted.