futuristiccivic

hey everyone so i have been looking for a solution to this problem ive had for quite a while now, let me begin with i have a 1995 civic ex with a b18a swap with the usual mods nothing too fancy. anyways my car tends to do magic with me sometimes the water dissapears after 2 days and sometimes it will hold it longer....4 days sometimes a week. never the less it somehow manages to loose the water. i canot find the leak there is no visisble nor obvious reasons for it to be loosing the water. any help on how to find out what is happening?
keep in mind i am using water because coolant is too expensive to be throwing out every few day. oh and might i add i took the thermostat out about 2 weeks ago so that was eliminated. and there doesnt seem to be water in the oil...at least i dont thing....the oil isnt really runny for how much water is has lost...
any help would be greatly appreciated



*******EDIT******
i changed the radiator cap left car running for about 15 min then the thing stated smoking where the resevoir is any idea????? do i have a bad radiator?