Very good point. I've even heard people say medium distance running is pointless for boxing. I completely disagree, if nothing else it can be great mental preparation.
About the dehydration...in general you don't want the fighter drinking water between rounds. You rinse the mouthpiece, he takes a mouthful and rinses, spits it out. I think that it was Jefferies that was adamant about not drinking any water during a fight, and those guys fought a lot of rounds. If you read old books about the prospectors that roamed the deserts in California and along the Arizona border hunting for gold, they always talked about not drinking too much water. The reasoning was that it left more to be baked out of you, so you drank enough but didn't overdo it.
I don’t think he did Ju-Jitsu for boxing. It’s literally pointless. I’ve seen Mayweather, Vitali and many more still chop wood.
My old pal Kirkland Laing told me he used to chew 10 pieces of gum at once for an hour at a time to strengthen his jaw. Not sure you’d find it in the boxers training manual 2022!
If I recall correctly, in marathons, overhydration tends to cause more serious issues than dehydration
Dempsey references chewing gum I believe in his book. Harold Johnson advocated it to Ali after a spar because Muhammad had a sore one