you need lots of water for many things
while it is possible to get too much water, someone who is working out and trying to gain muscle will have a hard time getting water poisoning
water helps flush urea out of the body which is a chief byproduct of protein metabolism, your muscles are also 70% water so its no surprise that some people choose to drink a gallon if they wish to increase muscle mass
also, the harder your routine becomes, the more calories you burn, and water can help retain that mass, as you dehydrate, catabolic hormones are released that eat away at all the lean muscle you've worked for
supplements also require increased water intake as well, with creatine, cell-volumization (fluid filled cells) allows for an anabolic stimulus which delivers more nutrients to your working muscles
again, take on as much water as you feel comfortable with, then increase if you choose to pump up your workout