Unless you can draw the water directly from the block (where the water would be hot almost instantly), you will be missing out on the biggest benefit of manifold heat, which takes place during initial start up. It all has to do with preventing atomized air/fuel from coming out of suspension when it contacts the cold intake manifold. Until the thermostat opens, there won't be any hot water coming through the heater hoses (if that is the source of your hot-water-intake-heater).
I don't know what the benefit is for continuous heating of the intake manifold once the entire engine reaches operating temperature.
Carl
