When water evaporates, it requires heat energy to do so, then the moisture carries off the heat it just asorbed, causing a cooling effect, same way your body self cools by sweating and then the sweat evaporating. If the humidity is high, you get less evaporation from the swamp cooler (or your body), and so it is less effective. The area where it is used needs to be fairly open to allow the now humid air to be replaced by are that is dry or at least, less humid, so I would keep the garage open while using one of these coolers.
This is why, people from the dry desert west, when in the deep south complain about the heat, even though it is not as hot (actual temperature) as where they are from, you will always hear the comment, "yes, its hotter, but its a DRY HEAT", and that is why your body stays cooler in a "dry heat".
Charles