Change the style! Not everything needs to be photorealistic if that’s not your cup of tea. Even if you don’t want to go toon style, underwater you can get away with solid colors + ramps especially with fancy shaders and effects like caustics. (Don’t ask me how to do it in BGE though )
Some hints if you still want to do texturing:
Texture paint brush with cloud texture can give you a good start. Use small strength, smooth falloff and have patience. Paint separately and layer multiple textures like base color and dirt.
Remember to use reference images, you can load them inside Blender in texture window or 3D view bakcground, they help a lot. You could even make your textures by taking a reference image and stretching the UV layout on top of that and then use it to bake a proper UV layout. Apply small fixes to it with clone brush.
You can bake lighting (SSAO/AO) to solid color texture which already bring much more life to your models.
Before you even start texturing, it’s also important that you have all the mesh details that you want.
Let’s say your model is finished. Now an also important step before texturing is a clean unwrap.
Fishes (and also the shark) have an overall elliptic shape, therefor it makes sense to cut them right in the middle and use a mirror modifier, so you get the same colors on the opposite side. some of the fins are not in the middle of the body, i recommend to mark seams for them, so they are seperate to the rest. also make the eyes seperate.
When you have space on your UV Map it’s a good choice to scale the seperate small details of the mesh. in this case the eyes (and maybe also the teeth). but dont scale the seperated fins, because you would get bad transitions on the seams.
Now it’s time to actually start with the texture :-).
On the first reference image i see 3 main colors, which you should prepair to work with.
light blue, mid blue and dark blue.
You can do this in Texture Paint mode of Blender or use an external software like Gimp. Start with the light blue and cover everything with it. now take the mid blue and paint it on the top side of the shark (maybe use a lower strength on the brush for smoother transitions). and finally finish it with the dark blue which appears on the edge of most of the fins.
Now bake the Ambient Occlusion of the Mesh and multiply the AO Texture with your painted texture.
If you are lazy just make the eyes pitchblack, it will look evil and is very easy. if you want it realistic, well either watch a tutorial of painting eyes or copy paste an eye of a good reference (don’t forget about license of the image; you may have to credit the author then)