Clouds - as you can see there are a few types. If they are going to be prominent/close in your image I'd recommend 3d, otherwise 2d will work.
Like water, you have an altitude, but you also have a depth.
Changes to variables here (Redsky, Bluesky, Decay etc) are noticeable quite quickly
Quality - Among the many (many) options for clouds is "Quality". Like many things it defaults to about rather lower - 0.43 for example -- setting it up to 1 will make a rather noticeable difference in the final image (more or less 'grain' at the edges), but won't actually increase render time by much. Not that clouds are all that long to compute anyway (compared to rough water)