I teach art at Mecca some Saturdays. Here are some photos of my lesson’s results. 
This is a nudibranch I guided an 8 year old boy to draw. I broke it down to basic shapes and encouraged him as he went.
This young girl is drawing a whale. 
This child appeared to put on too much water. Cool skull though.
A sunken ship! I didn’t prompt them to draw that. Notice the crystalline trees to the right. Or are they houses?
You can see the bright oil pastels show through the blue water based paint. Sometimes the students use colored pencil or crayons. 
Coral and layered paint.
This was an example I made to show them how.
After drawing fish, eels, sea anemones, sea turtles, jellys, coral and sea weed, the children paint blue over with sponges.
Some students abandon their drawings, so I recycle their pages to guide other students. I like to not finish my projects so that students don’t imitate me fully. I like this lesson because it doesn’t have to be changed for older kids, just expect more detail out of them. In most projects, for some reason, I get push back from kids when I ask them to put in more when I think they aren’t done. This project usually has less of that, perhaps because there is always more to put in the ocean. I’m going to mail these to my niece in Amsterdam to put up on the wall around her cradle. I hope they aren’t nightmare inducing.
I even got my mom to try this lesson.
Didn’t she make a nice scene with layers of paint? Her fish have some personality. You can try it at home or share with children.
Isaac and the Awkward Situations 