Get hydrated not tyred! A gym trip is hard enough so don't make it harder on yourself: keep hydrated.
How much should I drink at the gym?
First of all, drink before you go to the gym, drink during exercise and drink after.
The actual amounts depend on your age, weight, sex, height, intensity of the exercise and the surrounding temperature.
The NHS page on water drinks and health points out that choosing healthier drinks is part of a balanced diet. The advice in the UK is to aim to 1.2 litres per day. The full advice can be read here:
The idea to keep taking on water during rest breaks and make being hydrated part of your exercise regime. There is a simple wee test. If your urine is sparkling white like an Alpine stream; good to go! Rather than a dark yellow colour which probably means you are not drinking enough. Refill your bottle once a day and you are probably taking on enough water.