I don't have any personal experience with the stuff, but Tandy Carnauba Creme says it's water-based. It likely won't be particularly waterproofing. Even their own description (designed to get people to buy it, after all), only says water resistant. Personally, I'd use beeswax. The method employed by some SCAdian armourers I know is using a propane torch to carefully heat the leather (DONT AIM IT DIRECTLY AT THE LEATHER! Just wave it near the piece until the surface is hot but not close enough to scorch), rubbing warmed but still solid beeswax over the surface, re-heating to liquifiy the wax and let it soak in (again, not aiming the torch directly at the leather; uncontrolled fire BAD, m-kay?), rubbing with a buffing cloth, then repeating the process until the desired finish is achieved. This won't fully harden the leather, but it will keep it from being so susceptible to water. It takes some practice, but can produce very nice results.