Tim--you're absolutely right on the cooler not working in high humidity. It's all I use to cool our house here in Tucson and it works great -- until our summer rains hit. Then you'd swear it wasn't doing a thing. A few times I've gone up on the roof to check it, thinking the float valve had malfunctioned, but it was only the high humidity keeping it from cooling efficiently.
For those who can use swamp coolers to cool a greenhouse, the botany department where I used to work had a nice set-up with the cooler pads being mounted in the wall at one end of the greenhouse, and a big exhaust fan at the other. It pulled that cool air through from end to end, and really seemed to work well.