Here's how we did it:
UNDER THE SEA WATERCOLOR RESIST
Watercolor paper, approx 11"x15"
Oil pastels, select any light and bright colors, except for dark blue
Blue liquid watercolor paint
Thick watercolor brush
Water container & water
Water spray bottle, optional
1. Using oil pastels, draw an imaginary underwater scene, including any of the following: fish, whales, sharks, crabs, eels, sand, seaweed, coral, bubbles... and use imaginary colors and patterns.
2. Make your lines bold and solid by pressing hard as you draw, and go over your lines a second time to make a thick layer of oil pastel on the paper. Be sure to leave some areas on your paper white.
3. Dip your brush into the liquid watercolor, and let it gently glide right over your drawing. You'll notice that your bright oil pastels lines resist the watercolor, and will pop out from the watercolor wash. Use plenty of paint, and allow puddles to form and dry in place.
4. While the paint is still damp, you can create interesting patterns by using the water spray bottle to squirt a few random sprays on your painting. (Don't overdo this step, as too much water will dilute your blue paint.)
5. Allow your painting to dry flat.