I agree with (and have heard the same thing) your point about proximity to usable lake water. Seems the CL-415's are really effective in Canada as there is an abundance of lakes. In the American Southwest that is just not an option, partly the reason why the forest service hasn't bought any of those aircraft. The 747 however hardly seems like a viable solution given turnaround times, access to that volume of water, available runways etc.
Like filling a swimming pool with a garden hose, how long would it actually take to fill a 747 with 100,000 tons of water? A day? Two?