Those evaporative coolers aren't real effective when the humidity is high. You didn't give a location, but the use of "heat index" says you are probably in a high humidity situation. AC is MUCH more effective in that situation.

If it's crazy hot, I just put a shop fan blowing directly on me and dampen my clothes with water, not dripping wet, but enough to know it's there. Only 100% cotton t shirts and jeans or work khakis. Same principle as the swamp cooler, but far more effective, as it is directly on your skin. Have to wet down every 30 mins or so.