A while back I posted an underwater watercolor lesson that I did with middle school students over the summer.
Recently I did the lesson with one of my fifth grade classes. It took about 5 30-minutes classes. We used foam brushes dipped in two hues of green for the seaweed and fluffy brushes for blue washes. I also decided to have the students use watercolor colored pencils for the fish instead of watercolors due to my lack of tiny brushes and lack of time. We did a few practice fishies before we drew them on to the backgrounds though. The watercolor pencils gave the kids some good practice in color blending, adding layers, and drawing small details.
I think they were a great success!