Whether you're painting a mural, a child's bedroom, or on a canvas, use the same basic steps to paint underwater ocean scenes. For inspiration before you paint, look over a few photographs of underwater landscapes and fish to get an idea of what type of scene you want to paint. Gather your supplies before you begin and go as quickly as you would like---to finish in under an hour, or allowing for more than one session to finish it.
Sketch out a quick drawing on a sheet of paper to create a blueprint of how you want your scene to look. While this drawing will not be as detailed as your painting, you can include basic items such as large rocks, fish, or coral so you can make sure your picture looks balanced on the page without every item crowding to the same side.
Determine the background colour of your scene, which can be a light or dark blue, or even shades of green. The closer you want your scene is to the surface of the ocean, the lighter your background colour; deeper ocean scenes are a deeper blue.
Paint the entire surface of your canvas or wall with your background colour by starting with a light shade of your background at the top. As you work your way down toward the bottom of the canvas, add in more of your blue and/or green base to subtly darken the shade.
Allow the background to dry thoroughly. Make very light markings with your pencil on the painting, outlining background items such as cliffs, coral reefs, large boulders, or shipwrecks.
Paint in your background items as detailed or plain as you want them to appear. As you paint, remember the light source for your work is coming from above, so darker shades should be used along the bottom half and base of your landscape.
Let the background items dry before tracing light marks for your more prominent items, such as fish, dolphins, starfish, or seaweed. Paint these foreground items over your background where needed, keeping the shading of darker colours toward the bottom of the objects you paint.
Finish your painting by adding any further details, such as fish scales, gills, wood grains on ships, or texture of coral to the images you have created. Often, stepping away from the painting and looking at it from about 10 feet away will help you see where your painting may look sparse or need more detail.
Paint a few of your items on small canvases ahead of time to get a feel for how you like to work and how simple or complex you want your detailing to be. This should also give you a rough idea of how long it will take to complete the full project.
Make yourself familiar with the type of paints you want to use and how to clean your brushes after you are finished with a painting session. Water colours and acrylics can be cleaned with soap and water whereas oils require white spirit or turpentine.