RGB Level increased to 4 produce more contrast, brighter spots. Now apply this map as projector. When the light is ready go to Environment again and add Volume Light effect. Suitable Particle Size you have to find yourself. This will produce fast and nice effect of bubbles in the water.
Underwater scene makes no sense when there are no object in the water.
With only one light from top your objects will be too dark, so you can add some fill light from one or two sides. You can exclude the bottom from this light if it is too bright. Now turn Cast Shadows on for only one light. Remember that for lights with large radius you will probably have to increase Shadow Map Size and Sample Range. You can also see better quality rendering from my picture gallery.