Disclaimer: this is not my lesson. This is Cheryl's. She teaches at St. Paul's Episcopal Day School in Kansas City, Missouri, and she was kind enough to allow me to visit and observe one afternoon not long ago. She and Shirley (her cohort) write many of their lessons up for Arts & Activities, so this might be one you've seen in that publication, but I saw it in her room. She does it with kindergarten, I did it with second graders.
I started by having them draw 7-9 fish on a 9 x 12 piece of white paper. No eyes, just fish. Then they traced them with black permanent marker and colored them warm colors using neon crayons. They then did a blue watercolor wash over them (sorry, no photos of this step, but I'm sure you have a vivid imagination). Next they did their weavings using cool colors of paper. Then they used construction paper to add sand and seaweed.
Finally, they cut their warm fish out, added a google eye and bubbles (using the watercolor-painted paper).
In second grade, this was more of a review warm/cool/neutral colors and try our hands at weaving once again sort of lesson. Beautiful AND educational, that's what I call a WIN! I'm sure I'll get many wish-I-were-there sort of comments when I hang these beauties up in the hall later today!