The leather utilized to produce water-resistant watch bands is treated with fatty materials during the tanning process to block water from seeping in to the pores in the leather. Consequently some leathers are more resistant to water absorption and can be classified as waterproof. However, understand that leather is a porous material and is not intended to withstand exposure to water for extended periods of time.
Watchbands that are classified as waterproof or are tested to specific depths can be worn in the water, but will break down with extended exposure.