The key to doing this is to create a caustic effect.
Caustics are the envelope of rays diffracted by surface defects in a material.
A simple example would be the white shining light moving in stripes inside a water pool. Light gets refracted unevenly when it passes through the surface of the water and produces the caustic effect. It has to be done through a work around technique. Before we start to create our scene, you need the pattern of an actual water caustic to use as a gel to filter the light. This is the heart of our believable underwater scene.