I have been searching on info about aero bottles, or where to mount regular bottles. There is a ton of information out there. They have done various wind tunnel tests, and the results are very surprising. One of the best pages I found was here:

I was amazed to learn that having one bottle mounted on the down tube was more aerodynamic than having no bottles. But having a bottle mounted on the seat tube was less efficient. And having a bottle on both the down tube and seat tube was much worse!
I was also surprised to see that having those behind the seat bottles that you see on tri-bikes was also aerodynamically worse. Apparently the air flows over the riders back and hits the bottles.
Having a single earo water bottle such as the Profile Design Aerodrink System. This seemed to be the most aerodynamic, and actually improved the bikes aerodynamic properties. And the bottle holds 32oz of water. It looks like the bottle will work by itself with certain Profile Design aero bars. But will need an adaptor to work with other styles. The bottle with bracket costs about $25.
Another style of aero bottles that I have seen are the Profile Design Razor Bottle System/Bontrager Speed Bottle and Cage/Arundel Chrono Aero Carbon Cage and Bottle. These are all similar. I have read they improve the aerodynamics of the bike. But as much or more than the single round bottle mounted on the downtube? I haven’t been able to find wind tunnel tests of this yet. These also only hold about 20-22oz of water. They are also kind of expensive at around $50-$60. But would it be better than a 32oz round bottle on the downtube? A round bottle cage costs like $5, and I have gotten the bottles for free at races.
I haven’t decided anything. I will probably stick with my round bottle, and mount it on the down tube. I may look into the Profile Design Aerodrink System too. Of course, I found a review where a guy trashes this system too:
Who knows!