I suspect this question isn't stated fully enough, and also should probably be stated as a pure math problem rather than relying on any domain knowledge about ETF's. If you're minimizing overlap and want to buy as few as possible, and those are literally the only constraints ... then choose zero of everything.

You'll probably get better answers if you can frame this problem in terms of something more purely mathematical, and then ask a precise constrained optimization question. For instance it seems like perhaps we could model this by probability theory. Say X is a probability space with events A, B, C, D. And P(A)=.472, and so on. In this setting you want ... what? To select some combination of events to all hold simultaneously, minimize the joint probability, minimize the number of events, and ... maximize something? Subject to some constraint, like at least two events occurring?

thanks for your reply. I corrected my post. I don't want 0 as the answer. As an investor, I covet exposure to AT LEAST 1 water ETF. Feel free to edit my post. I afraid that if I generalize or broaden my question into a pure math problem, I won't understand it!