I encountered those electric showerheads in Honduras and Costa Rica. They looked pretty menacing, but I never got shocked by one. The key to getting hot water is slowing down the water flow just a bit. If it comes out too fast, the heating element can't keep up. The possibility of getting a full-blast hot shower is pretty much nil.
The on-demand water heaters we had in Europe were wall-mounted gas units. Much cheaper and more efficient than the big tank-types. I checked into them a while back and was really surprised at how expensive they are here in the States. Maybe there's a water-heater lobby that's afraid of competition.