Learning to draw people, animal etc is important but you also need to be good in drawing surrounding environment of your composition because that will add life to your work.
Creating under water environment: Adding different sized bubbles and small fishes around will make your composition look like an underwater world.
If you want to put your characters near ocean floor then in addition to bubble and fishes add couple of wavy lines with few dots to represent ocean floor. Add some sea creatures and some rocks scattered around to deepen the feeling.
Adding the sense of depth and perspective: in real world every thing is 3d, but your canvas is 2d. So to successfully represent real world on paper you need special techniques.
In real world objects close to us appear larger than objects that are away from us. We apply the same techniques to represent the depth of a scene on a paper canvas. To make this technique even more powerful you can draw objects that are farther from the viewer with faint lines.
Another thing to remember is you can always draw lesser details for the objects that are away from viewer.
Why shadows are important: Many beginners draw characters that appear to be floating in mid air. This can easily be avoided by adding shadow of your characters. Adding exact and accurate shadow can be difficult and time consuming, but fortunately you do not need to draw very detailed shadow. Adding small patches to represent shadow will work just fine.
For drawing outdoor scene plan the position of the horizon first: While drawing outdoor composition beginners often confuse the position of horizon. This can be easily avoided by deciding the position of horizon at the beginning before drawing anything else on your canvas.