Ok, it seems that immersion is a no go. What would be the closest, perhaps giving the leather
some sort of warm oil bath or similar? I guess in theory, you could replace all of the air in between
the fibres with oil or a pliable wax and lose some of the flexibility and suppleness to gain
water resistance, that's about all I've thought of.
Might get some scraps and "infuse" them in some warm, thin oil overnight and see what happens.
I know that purposeful immersion of leather goods will destroy them without a doubt, so for holsters
for diving and other watersports, leather is out of the question, my idea is to make something where
you could get caught in a rainstorm or accidentally drop the item in a pool etc and be able to salvage
the item with air drying and oil or wax treatment.