What factor significantly influences how objects appear underwater?

The appearance of objects underwater is significantly influenced by the density of water. When light enters water, it is refracted, or bent, due to the different densities of water compared to air. This refraction changes the way we perceive the position and color of objects submerged in water. The density impacts how light travels through the medium, affecting visibility and clarity. For example, in denser water, such as saltwater, light is refracted differently than in freshwater, which can alter the perceived distance and appearance of objects.

While the temperature of water and the color of water can also play a role in visibility and appearance, they do not have the same fundamental effect on light refraction as density. The size of an object may affect how much light it reflects or absorbs, but it is not a primary factor in how objects appear underwater in terms of optical properties. Thus, the impact of water's density is the most significant factor among the choices given.