For low volume use even cold-starting a 3-4 plate steam boiler isn't going to be any more efficient than a standalone tank during the summer months, but the net heating + hot water efficiency will be higher during the winter. Every time you fire up the boiler in summer you heat up it's entire thermal mass to well above domestic hot water temps, heat that then gets "abandoned" in the boiler, slowly dissipating into the basement. Just the ~400lbs of cast iron has the same amount of thermal mass as 40 gallons of water, and if you're only doing 1-2 burns per day your summertime efficiency will be under 30%: 18% of the source fuel energy goes up the flue during the burn, then 2/3 of the rest of the heat of the burn stays in the boiler rather than the indirect, and the boiler has a much higher standby loss than the indirect itself- it'll be pretty close to room temp after 10 hours. With heat purge control you might get it up to 35%, net efficiency by ending the burn early, and parking the boiler at about the same temp as the indirect rather than 20-40F hotter, but the thermal mass of the boiler is a drag on hot water heating efficiency when there isn't a heat load.
At very low hot water use an electric tank water heater might have a lower overall annual operating cost. It's a more expensive heat source per BTU, but much lower standby loss than either the standalone gas fired tank or an indirect on a small cast iron steam boiler. The installed cost of the electric tank is comparatively low too- half to 2/3 that of a gas-fired tank, and WAY below that of an indirect. If you're a 25 gallons a day kind of user that's probably the right way to go.