There are really two types of efficiency. One is how well you use to juice you do draw to heat the water and the other is how much juice you need to draw in the first place in order to achieve the same temperature increase. Existing electric heaters may well be close to 99% efficient as far as using the electricity they draw to heat water. And in that area it obviously would be very hard to improve. There can technically be no improvement really.
But there could be an improvement in how much energy is drawn in the first place and this would be mainly due to the way the energy is transfered from the device to the water. The heating elements in a traditional heater may have to get far far hotter than the water itself just because they don't come in contact with all the water at once, just a small layer of molecules. The magnetron will transfer energy directly to the entire mass of the water which might make it less wasteful. And once you switch the water off you are left with some very hot heating elements that have energy in them never used at the tap as hot water.
I guess using an electric oven vs a microwave to heat a gallon of water by a number of degrees is a fairly good example. They both transfer 99% of the energy they draw into heat but I'm fairly sure the oven will use more watts hour at the end and also take longer.
Maybe I am wrong as to the details but there definitely has to be some fundamental reason why transferring the energy as microwaves is better than through contact, otherwise there would be no reason at all to bother developing a microwave heater. Well other than some minor advantages like less corrosion. And they would know that.
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