You can do water proofing things to leather stretched across a frame like for a boat that you can’t do with leather worn on feet.
Just as a larded leather cape can keep the wet off a man’s back in the rain, for awhile.
But good luck larding or oiling the sole of a boot well enough to water proof it and still be able to stand upright without planting yourself on your tail.
rather than oil, wax is an excellent waterproofer
The Brendan Voyage (1976–1977)...Convinced that the legend was based in historical truth, in 1976 Severin built a replica of Brendan’s currach. Handcrafted using traditional tools, the 36-foot (11 m), two masted boat was built of Irish ash and oak, hand-lashed together with nearly two miles (3 km) of leather thong, wrapped with 49 traditionally tanned ox hides, and sealed with wool grease.
Between May 1976 and June 1977, Severin and his crew sailed the Brendan 4,500 miles (7,200 km) from Ireland to Peckford Island, Newfoundland, stopping at the Hebrides and Iceland en route. …
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Wax and tar are quite good. And you can make a remarkably flexible pine pitch mixture with deer or rabbit crap pellets for fiber, and so ground charcoal that seals, too, and sticks like fiberglass.
Not to mention if you use sinew rather than commercially spun synthetic thread, the sinew when wet swells to tightly fill the holes- no doubt certain plant fibers or animal hairs spun into thread will swell when wet as well.
That said, I doubt the guy was wearing hip waders. Thigh high boots were probably more popular for riding, and he probably didn’t have to live in his boots like a soldier.