To keep your body running efficiently, it needs to be fully hydrated. Water is perfect for maintaining you during normal daily activities but exercise can quickly push your needs into areas where performance can decrease without hydration.
Start hydrated. Make sure you have consumed a steady stream of water hours before you start and your pee is clear by the time you are going to lift. You don't want your body throttling your output.
While exercising, sip an electrolyte drink before you feel thirsty. Don't drink enough to get sloshy in the stomach. Your drink should provide a slow and steady stream of hydration at a minimum, and some carbs, protein, and electrolytes depending on the type of activity.
I like to mix in a powdered supplement with a ratio of carbs, protein, and electrolytes. If you want to make it at home, you can get mix maltodextrin, BCAAs, and coconut water. But it is easier just to buy a high quality power.
(Just avoid anything with corn syrup, brominated oils, artificial colors or flavors. You don't want your body choking on that crap).
Get yourself an electrolyte powder so that thirst during the workout doesn't become a mental or physical distraction.