An electric water heater efficiency is basically down to insulation not the technology of the energy transfer device. 1kW.hr of heat energy put into water is the same regardless of how you do it. Efficiency of most "devices" usually comes down to heat loss, if it's a mechanical work device then heat is usually undesirable, if it's a heater then it's all desirable. A bog standard immersion heater is pretty efficient...... and cheap and simple and reliable, insulate the container effectively and away you go.
As for the small scale local generators, it's usually a case of the size. Solar and wind have a typical power "density", e.g. solar in the UK latitudes is somewhere round the 1kW/m2 range (ballpark), if you want more power it means a bigger panel area. I don't know what the typical wind power density is (easy enough to estimate) but if you want more power it means a bigger machine. I don't know what power you could expect from a 2m tall domestic wind generator, I doubt it's much on average, it's just not very big.
You have to be a little careful in considering peripheral energy sources like absorbing power from passing vehicles. If you are not careful you end up with extra drag, or it's a hidden "perpetual motion machine" (use the vehicles motion to drive a generator to charge the vehicles etc). It depends whether you can harness the "waste" energy without influencing the source.
It's generally easier to try to reduce demand than improve efficiency. 10% reduction in demand means 10% reduction, simple. I reduce the total fuel consumed by my car by leaving it parked on the drive, it's remarkably effective.
