As far as I know, there are (maybe, were) two different standpoints in the US and Russian/Soviet Army(-ies). In practice…
One of my school classmates did military service in… a very hot climate and told me that they never were allowed to drink on the march/on the move – at all! Only before and after – and only warm liquid with a small peace of brown/black bread, salted.
American soldiers use special equipment to drink permanently – everyone who watches TV can see how they do that right on the move.
The same I see in leisure centeres – most local people permanently (~ each 15 min) drink-drink-drink water WHILE doing hard physical exercises. Frankly, I see it with trembling… but I know that this is widely spread (here) ‘doctors recommendations’.
I myself never do that. If I need to do something hard/physical in a hot weather for several hours, I usually drink a glass of salted/mineral warm water before. And after. Never in the process, as I exprerienced that it makes myself less strong and enduring. I definitely loss much of my power if drink while running (know that from my schooltime ).
But, maybe, my personal need in water is low (isn’t very ‘developed’ :)) – I normally don’t need more than 3 cups per day + and 4-5 fruit. And never carry with me that bottles of water as I can do without drinking all the day – there is no extreme need for me to drink during the day in the UK climate. And I’m not overweighted at all.
Buy the way, in the USSR, in so-called ‘hot’ plant floors (in steel-making industry) there were special water machines with mineral water – with additional special button signed ‘a portion of salt’.