Water bottles are traditionally carried on the frame. One on the seat tube and one on the down tube. This works fine if you are bent over and can reach them. If you are riding upright and don't bend over like you used to then mounting the bottles on the handle bar can be a good solution. This may not work for all handlebars but the handlebar on Katie's bike is the same as on my Schwinn.
One thing to consider is that not all water bottles carry water. I have water in one and an electrolyte solution in the other. I use Skratch Labs Exercise Hydration Mix. This becomes very important as the weather heats up and the sweat increases. Absolutely needed. Having the water bottles within easy reach is so civilized.
Some like to use camelbacks which are back packs that carry water with a tube to drink the water. Carrying weight on my back during hot weather is the last thing I want to do. During the 1988 Ride Around Mount Ranier in One Day I was carrying a day pack for food and clothes. It was hot and the pack didn't allow the sweat on my back to evaporate and cool me. Let the bike carry the water.
The water bottles are from Klean Kanteen. Plastic bottles are fine but these are stainless steel, wash easily, are easy to drink from, and last forever. And ever.
\The handlebar bag is a Lone Peak Mt. Baldy Handlebar Pack. It's not too wide so that it gets in the way of my hands when I'm riding on the taped part of the handlebar. It's big enough for food and tools. I usually carry my tools in a seat mounted bag. It hinges on the front side for easy access while riding.