The roof of the cavern is even with the top of the storeroom chamber that you occupy. The ceiling is 7 feet above the floor of the storeroom. Shining the lantern over the edge gets some benefit form the wet ceiling, which reflects on the scene below. The floor of the storeroom is about 20 feet above the bottom of the cave (the bottom that you can see).
The cave floor is filled with free standing water. The cavern's size is perhaps 30 feet across, at the ceiling ~ however, across from where you are, there is a slot about twenty feet wide, the roof of which is about ten feet above the waterline.
There seem to be a number of rocks that emerge above the surface of the water, but the water itself is black in color and you have no way to determine how deep it is. There is moss growing on the lower walls.
You don't see anything moving.