Here is a special product for that... http://capellaflavordrops.com/waterflavors.aspx
i have only used their regular flavor drops, which work well in lemonade but not in plain water. I have seen other water targeted products in a website for a well known company in the cooking world called <something> oils.
P.s. i would think of flavor secondarily to the carrier technology. These various vehicles of bringing flavor can be radically different in how they behave... look for compatbility to your goals (which may be difficult), then flavor assessment as a last detail.