You can easily create interesting textures and effects using different watercolor painting techniques, such as dry brushing. This method requires you to dip your dry brush in paint and create a thick stroke directly on dry white paper to give off a textured and rough look.
Similarly, you can use other watercolor painting techniques such as the wet-on-wet method to create soft transitions and salt watercolor techniques to create a textured background. You may also create a look of splattered water using the splatter method, or use the method of scratching with a sharp object such as a toothpick to create textures like grass or fur.