Nearly one third of the US population is estimated to be suffering from dehydration at any one time. People just do not drink enough water. The minimum you should be getting is about 8 cups per day, however if you exercise regularly you should drink even more. Staying hydrated is an extremely important part of any exercise regime.
Water is essential to cellular function. It regulates the body's temperature, lubricates joints, and protects body tissues. These functions are all highly relevant to a weight lifter. Your temperature will increase when lifting weights, and you need to be hydrated to keep your body's temperature in check. Your joints should be moving smoothly to get the most out of your reps, and again water is essential here. And finally your body tissue will be growing and expanding. As you challenge your muscles they will grow. If they are to grow effectively and at their maximum potential, once again you need to be properly hydrated.
It is easy to stay hydrated. Take a bottle of water with you to the gym, and have sips between sets. Your muscles will not be able to perform effectively without water, so give them what they need. Thirst is not a good indicator of hydration as we often become thirsty far beyond the point when we should have started to replenish fluids.
In addition to having a bottle of water with you at the gym, you should carry one with you all day, and routinely force yourself to drink in the morning, at lunch, and at dinner. Also remember that for every fifteen minutes of exercise your perform, you need to drink one additional cup. If you are to stay properly hydrated, you will need to take on board far more water than you did when you were inactive.
By paying attention to your hydration, you will improve the effectiveness of your exercise efforts, and reduce the chance of injuries.