When I go for a bike ride, I always wear a Camelbak and carry in a zipped pocket on it. I know it's slow to get to but it's better than nothing plus I like to have the extra water.
Its really not all that slow either, if you adjust your tactics a little.
Its also nice to be able to carry anything you need like a raincoat, or tools, tubes, etc.
They make a lot of different models in many different sizes.
If you want to loose weight, your on the right track with the bike. I lost about 60 pounds in three months riding mine. Have kept it off for almost ten years now. I still ride just about every day. In the summer, when I ride longer, you HAVE to eat well just to keep the weight on, and I still get to eat my bag of Hershey's Nuggets a day.