Water likes to find its own path, so where it's hitting the floor could be quite a way from where it's exiting your engine. How quickly are you losing it? As in, how fast is the header tank draining down?
Water pump, thermostat housing, pipe end perished, pinhole in a pipe, (in decreasing order of likelihood). If it's very central it's unlikely to be the radiator, but check it over anyway.
Look for "witness marks", which in the case of coolant will be white and powdery, tinged with the colour of whatever coolant you're using. If it's a pinholed pipe, you may see the residue on nearby bodywork or the engine itself.