A recent two-part Australian study investigated hoof moisture levels in horses exposed to wet versus dry environments, as well as the the impact of soaking a hoof on moisture absorption. 

Brian Hampson, PhD, member of the Australian Brumby Research Unit at the University of Queensland and lead author of the study, relayed, "The moisture content (of the hoof) is controlled by the internal circulation, which is fairly constant in the healthy horse." 
 
Hampson and colleagues collected hoof wall samples from routine feral horse-culling operations in New Zealand and Australia. They measured the hoof wall moisture content in hooves from wet, semi-arid, and arid environments. Moisture content was nearly identical in hooves from all three environments, averaging 29.5%. .
 
For the second part of the study, the team aimed to evaluate the common practice of soaking a horse’s hoof. Using six domestic horses, they placed each horse’s right forefoot in a dry rubber boot and left forefoot in a rubber boot filled with water for two hours. Directly after, they trimmed the hoof walls of both forefeet with nippers and removed a small section of hoof wall horn and sole horn with a hoof knife and evaluated the moisture content of both.
 
Researchers found that soaking made no difference in the moisture content of the hoof wall. It did, however, make a significant difference in the moisture content of the sole. "Because the sole is prone to bruising and damage when weakened by excessive moisture, soaking the foot regularly may in fact be detrimental to foot health," the study authors noted.
 
They concluded, "If the hoof wall horn moisture content does not vary between horses in different environments, then the application of moisture to the outside of the horse’s hoof may have no effect on the i