You should have a sky and an ocean depending on the subject matter you pick. It determines your colors and what you are going to paint. I don't know if you are doing sunsets, silhouettes, daytime, or sunrises. Silhouettes are good for sunsets. silhouettes of palm trees, people, boats, etc. but lets assume you did a daytime ocean and sky and you are going to paint a flying marlin. The subject matter will differ, your choice of fish, in this case we are using a marlin. Find a picture of a marlin in a magazine of any sort, it could be completely out of the water or half out of the water.
Imagine your marlin picture in a silhouette. In your mind place that silhouette on the ocean or halfway near the center of the canvas. In seeing the outline of the marlin, you can see how large you want it. You will begin with a white silhouette of your fish. Let it dry. Once you have the white silhouette of your fish, you can easily blend colors on a palette or paint solid colors on the white silhouette. Use a spatula or brush to blend them. Just doing the basic color structure of the fish. Detail work on the fish is last. Once you have the blended colors that completely fill the white silhouette, and it's completely dry. You can now start detailing the fish. Usually start at the gill fins and the anal fins and the fins on the bottom of the fish. Then find out where the eye is placed. Pay attention to the detail. On doing the fish's eye, start out with the outer ring let dry. Do the inner color and then usually black is in the center. Let each color dry before you do the next. The last thing done to the eye is a tear drop spec of white on the upper side of the eye.
Your dorsal fins and tail fins usually have different colors, if you have not filled in those colors yet do that. Let dry. Then do the bone ribs on them. A good place to find out the aesthetics of the fish is the Pacific Sportfishing books with illustrations to show you what the fishes look like. The fish colors in the books vary as do the artists. When doing the rib bones in the fins pay attention to the bone colors, sometimes they are none existent, I use iridescent white mist for those bones. Other times the colors of the bone fade from dark to light. They get lighter as they go away from the head.
You now have a fish flying or coming out of the water. in either case, fish that do this have water splash that rises with the fish. This is the final step. It needs to be tied in with the water. Use iridescent white mist for the splashes. And you can use glittering blues and silver in independent dots and a tiny bit of seafoam green. Where the fish comes out of the water use a little white, if any. It is easy to ruin with too much white. Tying in the splash to your ocean with extending foam ripples using iridescent white mist. To see an example of what you are going to create go the slide show. You can click on the pictures of my flying bill fish. These pictures explain the tutorial in color detail.
You can add caught fish with a boat somewhere near the horizon, putting feathers in the fish's mouth and a fishing line using silver glitter. Now you can add seagulls or anything that would fit on your picture. If you have created a wave on the bottom with sand, you can put dry grass in the foreground like my Sunrise Silhouette with palm trees. The palm trees were painted first and the dry grass second. Some of my ideas come from three or four magazines. I see a fish I like in one, a boat in another, a sunset in yet another compiling and using three or four pictures to create one. Thus, making it an original. If you have any questions feel free to ask by blogging me.