How do I create an underwater scene?

I dont think your scene actually needs water. If you want to make it look like its under water, you could use a shader.

Lots of things contribute to making a scene feel like it's underwater. You'll probably want to use a strong, dark, blue-green fog (ideally with volumetric lighting), a weak directional light from above and blue-green ambient light. Then you can add additional effects like fake caustics or maybe a "wobbling" distortion effect. If you need help with anything specific, I can go into more detail.