Brian Hampson, Chris Pollitt and colleagues at the Australian Brumby Research Unit at the University of Queensland, completed a study in 2010 in which they sought to determine the affect of various environmental conditions on the degree of hydration in hoof horn wall tissue from feral horses. They also looked at the affect of short-term foot soaking on moisture content in hoof wall/sole tissues.
They utilized 40 feral horses from three Australian environments — wet/boggy, partially flooded and dry desert — and six Quarter Horses.
They found actual moisture content of the hoof wall did not differ in the various environments, but soaking in water increased the percentage of moisture content in the sole.
They concluded that the hoof regulates hoof wall moisture from within. Topical application of moisture doesn't affect hoof moisture content. This makes sense, because horses and other equine species are adapted to semi-arid environments and have no biological need for external moisture application to maintain hoof health.
Being constantly in water does make the sole softer, however. The researchers concluded that the sole is more porous and more readily affected by a moist external environment, which can leave it more vulnerable to bruising.