I bought a pair of boots recently, that after a few hours wear, I began to think I'd got the wrong size. So I dunked them in warm water while wearing them, then wore until dry. I now have a pair of boots that fit like slippers.
Of course, getting yours that wet may not be suitable, depends on the leather.
My old grandad used to be a postman, and he always swore by wearing his new boots and walking through deep water, and wearing them till they were dry. Always fitted perfectly after that.