If you use electric resistance heat to heat the water in the tank and then use a heat exchanger to extract it, you will use more energy than if you just used electric resistance heat in the first place.
There is no "free lunch". Smarter people than you and I have tried to increase efficiency of heating systems.
After two reverse cycle AC failures (my boat is in warmer water than yours but the raw water can still hit 40 degrees, I installed an AC with built in electric resistance heating, not reverse cycle. There's no raw water input in heat mode and no reversing valves, etc.
It's not as efficient as reverse cycle but I pay a fixed rate for electricity and replacing the AC units was getting expensive.
If you really want to try heating water in an external tank to run a heat pump, think about solar.