Refraction is the tendency of light to bend when it passes from one medium to another, such as from water to air. It turns out that light travels at different speeds through various substances due to different densities.
For example, light travels through air at a faster rate than through water, so when light enters water, it enters at a different angle. Unless, of course, you’re coming in at a pure 90 degree angle, like the sun when it’s directly overhead.
So in order for divers to see clearly underwater, we use masks. Now light must travel from water, glass, and air to reach our eyes. During each pass, light must bend, shape through each medium before we see the object. As a result, objects will appear closer to us in a 4:3 ratio than their actual size. Which means that an object that is 4 feet away will appear to be only 3 feet away. This is why it is difficult to determine how far away an object is in relation to you as a diver, as explained in the next paragraph.
There is a tendency to underestimate the distance due to refraction. At greater distances and under certain conditions, objects may appear farther away than this ratio suggests. This phenomenon is called visual inversion, where objects appear to be further away than they really are. In other words, visual inversion is just that, the opposite of what you would expect to see for an object on the ground.
This inversion is depth dependent and appears to be the result of decreased brightness and reduced contrast. The deeper you dive, the less light is allowed to penetrate the water. So it gets drastically darker the deeper you dive. The same thing happens with color, the deeper the dive, the less color appears. As a side note, this is why many divers carry lights underwater to see colors, even if it’s bright on land.
It can also be the result of the absence of visual or distance cues like the ones we have on the ground.
In very cloudy water (low visibility), even relatively close objects tend to be seen further away.
As a final rule of thumb, when estimating distances, the closer an object is, the more likely it is to appear closer than it actually is due to refraction. The cloudier the water, the more likely it is that the nearby object will appear further away than it is due to visual inversion.