I always thought it has to do with the way people think of things. When you look at a patch of ground, you see rocks, sand, grass, sticks, roots, seeds, etc. A lot of individual things. When you look at the ocean, you think "ocean" or "water" or "sea". A singular entity.
So, since Shadesmar is the realm of thought, it makes sense that bodies of water are solid and landmasses are beads.
And yes, I'm pretty sure that the Cognitive Realm reflects the Physical Realm in the same way on other worlds too.