Is your AC reverse cycle? If so, that should provide heat.
Sorry, but it simply will not.
Why?
Because reverse cycle "heat" is like turning your window air conditioner around and blowing the warm former exhaust heat INTO the house.
In reverse cycle air conditioning systems, instead of turning the window unit "inside/out" it is performed by a reversing valve inside the unit.
However, what happens in reality is that heat transfer is based on temperature differences.
When the water temperature gets below around 40F the heat stops.
I won't bore you with the details, but it comes from one of what I know is ALL OF YOUR FAVORITE classes:
THERMODYNAMICS.
This is also why many rev cycle AC units ALSO have electric resistance heaters in them.
Either electric heat (resistance, in the unit or separate; or oil filled) or diesel heat (like John's system) works. Others have plumbed in bus heaters (a coil with a fan) to their freshwater engine coolant lines, like your water heater), but this only works when the engine is running.
--- My 45 year career was as an HVAC engineer.