Describing words, to help you tell someone else what something is like.
It is dark when there is not much light and you have trouble seeing anything. It gets dark at night time, or when you close the curtains and shut the light out. The opposite of dark is light or bright.
Dark may also be used to describe something that has much deeper colors than something else, or is close to black. The opposite of this kind of dark is pale.
Water is called deep if you cannot touch the bottom with your feet when you stand in it. Deep water can be found in the sea, or at the diving end of a pool. Wells and mines are very deep but don't always have water in them.
The man in the picture is diving in some very deep water.