The comfort of waterproof footwear is dependent on it supporting feet and being breathable. The structure of leather and therefore the level of support it provides is maintained by tanning agents.
When leather absorbs water, it softens and tanning agents are lost. Subsequently, the leather is stiffer when it dries and therefore prone to splitting. Also, when leather gets wet the breathability of footwear is impaired, giving you damp feet.
Therefore, it’s essential that the Durable Water Repellency (DWR) of your footwear is maintained. Eventually DWR will wear off and need to be renewed with Nikwax Waterproofing Wax for Leather™.