For purely resistive heating, Y can approach X, but never exceed it.
....it can't be exceeded no matter what method you use - in the case of the OP's question, induction heating would be one of the worst and least efficient!
What you've done is extrapolate the OP's original question to include PRE-heating of the water to reduce the amount of energy required to increase its temperature anyway - which is why I joked about using bottled
boiling water to start with!
On a
straight energy-to-hot water conversion process there is nothing simpler than resistive, nothing more efficient (on a stand-alone basis) where any losses are purely thermal leakage. A heating coil in an insulated tank is the standard example - and very efficient.
We see a lot of advertising for electrical heating systems that claim they are more efficient than whatever but in fact all they are doing is adjusting the time and process of creating, storing and/or releasing the generated heat - they can't (and don't) do it any more 'efficiently' as they claim they can.
If you want 1kW of heat you use 1kW of energy (disregarding losses) and no 'method' will get you that 1kW out without the 1kW in - on a purely electrical basis anyway. The snake oil salesmen that claim their 'new, improved' heating systems can be more efficient (????) are con men, pure and simple.
You might get the heat at a time and place of your choosing but you won't get that 1kW out without putting 1kW in - end of.