The time when athletes were forbidden to drink water is long gone. There is now research to show that dehydration, per se, does not occur with moderate training. On the contrary, the chemical reactions of glycolysis that occur during training have water molecules in the final reaction and its level in the body increases. If we take sports, then, of course, everything is different there. Have you watched American bolibuilding movies / videos? They all come with 5 liter cans of water.
Points of view regarding taking water during training differ, there are those who believe that drinking water during training is categorically forbidden, but this is not really so - drinking yourself - yes, not the best option, because from the load it can be banal vomiting, but drinking small sips between approaches in strength training, for example, is very necessary. Just like dancing (any other cardio). The body, along with sweat, loses a lot of water, the blood acidifies, from which dehydration occurs, and corny it can become bad (fainting may occur) from the same dehydration.