I think you are unlikely to find a place where the waterline is significantly higher than elsewhere; the original post introducing the term made the point that the waterline must be depressingly low even across science for a theist to win a Nobel prize. A dedicated community like Lesswrong is about the only place where you can find that kind of thing, and last time I checked we haven't established a utopian rationalist commune in the real world yet (though I see the idea has come up before; amusingly enough).

As for places where there is a higher proportion of LW-level or above people than in other locations, it seems to me that, in the absence of explicit rationalist training for their people, locations with a high proportion of intelligent beings who are trained in empiricism and the use of mental models of reality (e.g. polytechnic university towns) would be promising starts. Another, more direct approach would be to see where LW members live, and either try to spot some trend or move to an area where they seem unusually concentrated. Do we have one of those Google map thingies where people of a community can post their locations? If not, we should. It would probably help with all those meet-up events, too.

Well there happens to be an LW conference in California coming up... Perhaps you mean longer term, though.