Drinking water during exercise is a very controversial topic. There are many opinions and many strong supporters of the opinions. Personally I do ocassionally I drink water during training and ocassionally I do not. It depends on the duration, intensity, general feelings and temperature outside, in the end it depends on whether I want to drink or not. Two-three years ago I drank more water while intensity and volume were lower. Now I want to take a sip only after a very intensive set, for instance, after a maximum pull-up set (goal – 30, I can do easily 20-25). Thus I think there is a connection between training volume, intensity and fitness level and thirst. That seems quite logical: the easier the load the less energy you spend.
While my own thoughts on this issue is not enough. So here is an opinion of a scientist, Svetlana Runenko (docent of the department of physical therapy and sports medicine of First Moscow Medical University I.M.Sechenova):
You have to drink at least 1.5-2 l of liquid daily. If you train then you have to drink even more than that, especially in summer or during hot weather. During exercising your body loses not only water, but also electolites what can lead to internal organ malfunctions.
There is an ancient belief among old-school coaches who forbid to drink water during training. There may be very special exceptions (if we are talking about elite athletes and competitive sports) to the rule when you really should not drink a lot of liquid, but you still should drink while you train, taking 100-150 ml every 15-20 minutes.
The taboo on drinking is often associated with losing weight. There is a belief, based on the fact that body fat contains 90% of water, that losing water helps with slimming down. This is an illusion. The same rule applies to excessive sweating in sauna: it does not make you slimmer, it only causes temporary dehydration. The amount of fat cells remains the same. Torturing yourself like this does not help, quite otherwise, it might lead to ruining normal function of the body: to drops in blood pressure, heart rhythm issues, blood viscosity changes.
To replinish your liquid loses you do not need special drinks, just plain moderately mineralised water.
I think this is a quite good answer, although leaving some white spaces. There are might be different approaches to drinking while training but there is a very common piece of advice: do not drink much in one take! First of all, you might want to drink more than you think because of the dry mouth. Secondly, too much water in your stomach might make your stomach feel heavy and uncomfortable and this might impede your training. Thirdly, it might overload your heart. That is you'd rather follow the advice given above and drink in small gulps.
Thus, I would recommend to drink 1-2 small gulps during your 30-60 second rest between sets, just to refresh yourself.
A piece of advice from Sergey Ilyukov, a sport doctor, who works with many elite athletes. He recommends to take notice of the colour of your urine. The urine should be clear and of light straw colour. The darker the urine the higher the load on kidneys and higher the dehydration.