Bought some shoes online and they're too big. Already worn them so can't return them. Tried them on in the shop, seemed to fit nicely but they feel too big in the real world. The shoes are the right width but too long. I can feel my feet sliding longitudinally.
Things I've tried: insoles and stuffing some socks into the toe area. Insoles didn't do anything and neither did the socks, maybe I should try more socks? I also read you can try shrinking leather shoes by immersing them in water. I don't think this will work with mine because they're waterproof also I don't want to totally ruin them.
If I take them to a cobbler or a haberdasher is there anything they can do?
No. If the shoes aren't visibly worn take them back to the shop. If they are (and you can get away with a TINY scuff or two) whack them on eBay. It is never worth trying to make poorly fitting shoes wearable. Next time pay more attention when trying them on, and always do it at the end of the day.