Image via WikipediaEvery scuba diver has been taught that underwater objects appear to be only three-quarters as far away as their physical (measured) distance. For objects subtending small angles at the mask and eye it is certainly true that, for any object distance, the object's in-water virtual image produced by the planar air-water interface (mask faceplate) is always nearly 25% closer to the interface. This is a consequence of the laws of refraction and is independent of the observer. It is not true, however, that the virtual image is always perceived to be at that distance.
Research into underwater distance perception contradicts the popular notion held by most scuba divers that things always look closer under water. An initial investigation by Ross (1965) found that object distances less than 39 ft. were indeed underestimated, as one would expect from the laws of refraction, but those greater than 39 ft. tended to be overestimated. Subsequent studies by Kent (1966), Luria et al (1967), and Luria and Kinney (1970) were in agreement that distance estimates for objects closer than one meter were, in fact, too small, but beyond one meter the perceived distances were too large!! Moreover, the distance overestimates increase both with object distance and water turbidity.
The correlation with water clarity suggests that the overestimates for large object distances are caused at least partly by loss of object-to-background contrast due to the absorption and scattering of light by water particles along the light path. The loss of colour and the haziness means that foreground objects are less distinct relative to the background. Studies in air (Fry et al) have demonstrated that reduced contrast effects cause object distances to be overestimated, so this trend apparently carries over into the underwater environment.
This cannot be the full explanation because the overestimates also occur in water even under conditions of high contrast. Luria and Kinney have pointed out that the major difference between scenes in air and scenes in water is that underwater there are relatively few clearly visible, familiar, and well-defined distance cues. The scenes approach the hazy, unstructured, homogeneous aspect of what is referred to in psychology as a 'Ganzfeld'. The characteristic loss of peripheral stimulation associated with a Ganzfeld impairs both object detection capability and depth of field judgments (stereoacuity). Experiments have shown that this leads to distance overestimates.
Hence, the underestimates that we all expect on the basis of refraction occur only at small mask-to-object distances.