questionshas anyone built and outdoor thermal type water…

vote-for20vote-against
vote-for4vote-against

Solar water heating doesn't do away with the water heater, but it certainly lessens the need for it to fire up.

I have no experience with it myself, but a quick search yielded this:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm

Reusing your existing water heater for a storage tank (from the solar heater) and adding a tankless for the boost (when necessary) would probably be your most energy efficient route.