First of all, create your overall scene (don't worry about making it underwater yet):
- Change your plane into something natural which you would find under water (i.e. sand)
- Add a Terrain and erode it to look natural and weathered, then apply the proper texture
- Add some items that you would find under water (i.e. anchor, fish, dolphin, etc.)
- Import and texture your Sculptris Underwater Creature that you built last week.
- Add a water plane and move it up above your scene to create a water surface.
- Try different skies until you find one that looks more realistic.
- Next, add some haze and fog and adjust the colors accordingly.
- Finally, add a spotlight and a filter to make light ripples.
Below is a gel image:
