110/220 wiring

No, that is not going to work (or at least not how you describe it) because, yes they need to be on opposite phases. You should already have 240V power in your kitchen for the oven, just use that.

Yes, I have 240V available, but the water heater only has one Line terminal.

I'm told I can land L2 on the Neutral terminal as the other phase, and disregarding the neutral as in most 240V devices. What do you think?

The way a typical house is wired is that you have +120V to ground coming in to the right side of your panel and -120V to ground coming in to the other side. To get 240V, you wire across the two and go -120V to +120V. On any given 120V outlet in your home, you only have one side or the other plus the neutral. No box should have both the -120V and the +120V in it unless it is a 240V box. This is one for an electrician. You need a 240V outlet at your water heater and a 240V plug on the heater, then you can just plug it in. If you want to do it yourself, here is a guide: https://www.hunker.com/13414163/how-to-install-a-240v-electrical-outlet

Thanks, but I understand about single- or split-phase wiring. The question is about how to wire this water heater that has only one spot for landing a hot wire, not two like a standard U.S. appliance. The specs are for 220V and 50 or 60 hz so it will operate in a variety of countries. The Philippines, where I bought the water heater, is 220V@60hz, the U.K. is 230V@50hz; both supply the voltage with one hot wire -- true one-wire single-phase, not the U.S. version of two-wire split-phase. The heater is set up to be hard-wired, not cord & plug. It also has a 4-position power switch (off and three heat levels), with two power lines to the element and I guess neutral/negative wires to anodes or cathodes all into the top of the tank. Is it as simple as landing L1 on the terminal's L screw and L2 on the N terminal? Or would answering that require a diagram of the heater's inner wiring? It's an undersink model, so all that wiring is exposed.