Light is scattered underwater by small particles suspended in the liquid. Individual photons are deflected or diverted when they encounter suspended particles just as they are in air when the atmosphere is full of dust, but the effect is much more pronounced underwater. This is not only because all kinds of particulate matter are typically held in suspension in water, but also because light is scattered by molecules of the water itself. Transparent biological organisms may make the situation even worse.
The effect of scatter underwater, once again as in dust-filled air, is to reduce contrast between the subject of an image and its background or surroundings. The same phenomenon affects both photographic images and human vision, in the latter case restricting visibility to much smaller distances than would normally be the case in air. The effect can be so pronounced that relatively large object may remain invisible at distances of only a few metres, but the situation also deteriorates rapidly with increasing distance. Perception of small details is also much poorer in water than in air despite the magnification produced by refraction in water.
One consequence of the severity of underwater scatter is that divers are obliged to approach their photographic subjects as closely as is possible or safe. They therefore choose wide-angle lenses that can focus down to short distances to create close-up images of subject that seem further away - the opposite of the technique used above water where telephoto lenses are selected to bring objects closer.