Well you should definitely try and match the color of your light with that of the water..

Then it would help to put an infinite slab with a water texture, and have it surround your scene. the top of the slab will be your water surface, the bottom of it could be below the ocean floor.

Further more, water tends to be more murky then air, so the haze should be stronger.

Those should cover the basics i think.. the rest would be a matter of tweaking things to your liking.

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