Diffuse lighting, lots of environment fog, depth of field, fake caustics for lighting, particle system to make bits of debris float around.
When looking for something similar, I was able to Google some animated textures that could be used for caustic-like lighting patterns. You should try doing the same.
Wait, I just re-read your post... if it's literally 'bottom of the ocean,' i.e. no sunlight, then you don't want caustics. Definitely have some volumetric light cones for any spotlights in the scene, though. YouTube is full of reference material, I'm sure.