Painting ocean waves on the walls in a child's room is a fun, creative way to play up a number of themes for either gender---mermaids, ships, pirates or even a general beach theme. Executing the task won't be as difficult as it may seem, especially since a stencil and two colours of paint are all you will need to create the ocean waves. Just make sure that each wave is level with the others to produce professional-looking walls.
Remove all posters or frames hanging on the wall you wish to paint. Remove any furniture that is in the way, and tape some plastic over the carpet to protect it from dripping paint. Wipe the wall with a damp cloth to make sure it is clean and ready to be painted.
Pour some light blue paint into a paint tray, and paint the entire wall with a few coats using a paint roller. Allow to dry completely.
Use thumbtacks to secure a wave stencil to the wall, starting on the left side. (Find a free, printable wave wall stencil at http://disney.go.com/disneyhome/nemo/images/nemo_wavestencil.pdf.) Colour in the wave shape with dark blue paint, using a paintbrush where the roller won't fit. You can paint the entire lower half of the wall dark blue or make the wave more of a strip design that ends where the wave does. Continue to work your way along the wall, moving and reattaching the stencil whenever necessary.
Add some sea foam to the crest of the waves using a sponge dipped in white paint. Dab the sponge in a stamping motion all over the inside curve of the wave as well as in random places in the water in between each wave. Allow to dry.