I have a boiler with tankless coil for hot water. I was using so much oil in the summer to keep the boiler hot that it cost more than if I heated the water in the summer with electricity.
I installed an electric water heater in series with the tankless coil.
In the winter, the water passes through the tankless coil and into the water heater. The water heater is set below the temperature from the boiler, so it uses no electricity.
In the summer, I shut off the boiler and the electric heater takes over automatically.
You can buy an electric water heater for a lot less than an indirect heater. Unless you have a gas boiler with condensing unit, you will probably cost more for fuel to keep the boiler warm than it will cost to heat with electricity. Feed water is warmer in the summer if the source is surface water. It doesn't vary much if you are on a well.
If you put in at least an 80 gallon heater, you can sometimes get a special rate from the electric utility if you allow them to control the heating hours.
If you are using natural gas, you may want to use a gas heater, with or without power-vent. I don't have any experience with that, but others on this board may suggest something.