When you're blasting down the Kona Highway, you need a water bottle that's not going to slow you down. So you should probably go with the Profile Designs HC Mount. It's a handy dandy little device that lets you put your water bottle between the aero bar extensions for easier access and a little less wind resistance.
Lightweight 6061 alloy crossbars mount to your aero bar extensions
Attaches to any bar extensions up to 130mm in width
Can be mounted above or below the extensions to suite your needs
Increases aerodynamic efficiency and decreases wind drag