Are you aware that water is formed as condensate from all heat exchangers that cool the air passing over them? Refridgerators, freezers, aircon systems all work the same. Whether you call the water stagnant or something else, the odour comes from the mould that grows in it. Most of the green moulds are harmless but smelly, the black and red moulds are less innocuous. Either way the solution is drain the water from the AC as it was designed. Do that and you won't need bombs, perfumes, or other chemicals.