Agree with above. If you're able, up the milk offerings and try to stay cooler during the day when you can - maybe some time in a small kiddie pool or the bathtub?
If push comes to shove and you feel you NEED water, I would honestly introduce the TINIEST possible amount in an itty-bitty cup or spoon rather than a sippy. We used mini-shot-glass-type cups we found in a play dishes set from IKEA and Natalie was able to manipulate them - just a couple drops of water in each so she couldn't make too big a mess at any one time. We'd line up maybe 6 of them each with a few drops for her to practice with - it looked like our baby was doing shots! LOL By the time she was maybe 9-10 months old she was an old hand at drinking from cups already, from practicing that little bit at a time.