Nimbostratus is a dark gray cloud associated with continuous rain or snow. The intensity of the rain is usually always light and is easily confused with altostratus. Only easy way to tell the difference between the two is that you can see the sun through an altostratus cloud but you cannot with a nimbostratus cloud. Stratocumulus clouds are gray and lumpy in appearance, they usually cover the entire sky in patches or long rows with blue sky shining through them. They most often appear during sunset and rarely form rain.
Stratus clouds are gray and uniform in appearance.
They are often confused with plain old fog, but it differs in that it never touches the ground as does fog. Stratus clouds produce a drizzle or light mist, never an honest rain. Most often resembling a cotton ball there are many types of cumulus clouds to be aware of. But as I said this is not a meteorology course and you are not meteorology majors, so we will only discuss them in brief using the chart below. No seriously it is of extreme importance that a boater have some knowledge of how to measure atmospheric pressure.