Although physically speaking an instantaneous water heater transfers energy from the boiler to the service water through a heat exchanger, a delve into the EnergyPlus Input/Output Reference guide shows using a WaterHeater:Mixed instead of the heat exchanger shown in the plant loop image in this question. The guide suggests setting the volume of the mixed water heater to 1 gal (0.003785 m3) and set the Heater Control Type to Modulate to model an instantaneous water heater. Heater Fuel Type can be chosen, such as Electricity or NaturalGas. The WaterHeater:Mixed can also be placed in a Boiler Plant Loop, replacing the heat exchanger shown in the image in this question.
When using the WaterHeater:Mixed object, I discovered that the Water Systems energy consumption is identified in the OpenStudio Results Report in the Annual Overview End Use table, as well as the accompanying Pie Chart.
Since posting this question, the End-Use Subcategory "Water Systems" is reported in the EnergyPlus output report as a separate line item. The Boiler is automatically assigned to the major category "Heating", whereas a storage tank is assigned to the major category "Water Systems". There appears to be no direct way for the boiler load to be assigned to another major category. To have the Water Systems sub-category show in the OpenStudio Results report, it appears that it would require a custom modification to this report code.