If it's a shorter ride I get away with a bottle or two. On the 100mile ride I just did this past weekend, however, I had a 2.5L camel back full of water, plus two bottles with other stuff mixed in with the water. I believe people who do longer rides with just bottles are filling them up along the way, but I prefer not worrying about finding a place to fill up, or dealing with the bike and shoes at a convenience store stop.

I had some gels and a cellphone in my jersey pockets. I had a spare tube, tools, and some food in the bike bag under the seat.