I came across this image, posted by user MsOctober1017 on Gab. Click it for a larger view.
They’re Undulatus Asperatus clouds. I’d never heard the name before, and certainly never seen them myself. They were added to the cloud “catalog” as recently as 2009. The Cloud Appreciation Society says:
“Since they look like the surface of a rough sea from below, we call them the ‘Jacques Cousteau cloud’ in honor of the legendary French diver and ecologist.”
They certainly do seem to resemble photographs and video that I’ve come across elsewhere of the sea surface from below.