Best way to hide the area not covered by the skydome?

Hi, I’m making a large map where most of the area is just water, which means there is a distinctly visible gap between the water level and where the skydome doesn’t come down low enough.

I can’t find any way to lower the boundary of the skydome, and usually I’d manually hide it with hills and foliage etc, but that would look completely wrong on this map since it’s supposed to be the middle of the sea, and would also be a lot of work.

Does anyone know of any simple, easy ways of covering it up?