One of the most difficult aspects when designing digital imagery is the creation of the surface textures for the various elements in the composition. You have to decide, in the earliest stages, what type of look you are trying to create for the project. This decision gives you the basis to determine what level of realism you have to adhere to, and therefore the distance that you can stray stylistically from the norm.
In the image at right, I was trying to capture a scene of a 
great whale rising from the depths of the sea after reading about a new fossil discovery in Peru. Whales are typically encrusted with all sorts of surface parasites, and so I knew that I needed a rough surface texture to start from. I keep a library of images from Nature that I have photographed over several years. Tree bark, mushrooms, flowers and leaves provide an array of colors that would be difficult to duplicate without the original subject matter. I chose the image below for its wonderful scaliness, and simply started experimenting from there.
The water was created by adding digital “noise” to multiple layers and then applying photo filters (through the application) to soften the graininess in those selected areas. Moving beyond that, I adjusted the hue and saturation to provide more of a blue surface reflection, and then changed the opacity of the various layers to either bring them forward or lessen their emphasis.
Since I needed more than one type of texture for the whale’s skin surface, I relied on the “Liquify” capability of the software to create the smooth grooves on his underside. The finishing touches included duplicating and replacing the “eye” with the original selection, as I felt that the contrast would make it more prominent.
There are so many methods for creating the elements that I have included here that artistically it comes down to utilizing those techniques which are most comfortable for you, but experimentation is always a good thing. Each designer has a style that is recognizable, and reflects his or her specialization. Perhaps that best projects are those that ask us to go beyond what we are now familiar with. That is how new styles get their start, and the time to start is now.