This is a great lesson I found in a 2007 edition of "Arts & Activities" magazine. It was created by Art Teacher Judy Kalil from Florida. It's a seascape lesson with a twist- the seascape is inside the underwater creature's body, so it's super fun and imaginative for students!
I've done this lesson both as a watercolor and with colored pencils (perfect for a sub lesson).
So students started off by researching and choosing an underwater creature to draw. Fish are more or less the simplest and most popular, but you can also look at whales, sharks, octopuses, etc.
Draw the main outline/shape of the body. Don't add any details inside the body other than an eye and a mouth. Draw some sand along the bottom of the ocean and a few little seascape-y accessories.
Inside the body, draw the main details of the seascape. We brainstormed ideas of what one might find in a seascape (boats, pirate ships, islands, palm trees, coral, etc.) to help give a starting point.
Once the drawing is complete, outline everything with a permanent black marker/pen.
Next class period: crack open the watercolors!
We painted the inside seascape first and then the background last.
Completed watercolor examples:
Colored pencil examples: