Paint the entire 16-inch by 20-inch canvas with low-viscosity gesso, using a large brush, and allow it to dry. Gesso gives you a smoother surface to paint on and prevents paint bleed.
Paint yellow sunbeams streaming down from the top. Fade them at the edges by thinning the paint with water. Dab with a damp sponge to remove some of the paint or water, if needed. Allow the paint to dry.
Create a wash by thinning down blue acrylic paint with water. Cover the entire canvas with the wash. Remove the wash from the middle of the sunbeams with a damp sponge. Allow the paint to dry. Apply a second coat of wash if you would like the background to be a darker shade of blue.
Paint blue waves at the top. Add rows of white waves and highlight with thin lines of shining silver waves. Paint an arch around the top of the canvas, so it looks as if you are looking out of a cave to see the ocean. Paint boulders set together like bricks. Cover them with flowers or vines.
Paint a golden treasure chest with a thin black outline. Fill it with gold coins, strings of pearls and bright gemstones in red, green, blue and purple.
Paint mounds of tan sand covering the bottom. Paint long thick blades of aqua-colored grass in the sand. Add huge iris flowers in red and purple hues with very short stems. Outline the flower petals with a thin line of white.
Create ocean life that looks friendly. Make a cartoon mermaid with a grass-green tail wearing a pink bikini top. Paint an aqua-colored whale that is rounder than usual. Make a blue-and-white dolphin that is shorter than normal and give it a crown or halo of pastel-colored flowers.
Glue three-dimensional objects onto the canvas. Glue craft eyes onto the ocean life. Glue tiny gemstones and a miniature string of pearls flowing out from the treasure chest. Glue embossed gold-foil starfish and shells in the sand. Stick a puffy sticker of an anchor or a turtle to the sand.
Frame the finished canvas or paint around the sides with a solid color.