The answer is pretty simple, actually. Ice is the way to go but forget about the stupid water bottles. Ice in a water bottle...even an "insulated one"...last for a few minutes at best. In a Camelbak, 7 or 8 lbs of ice will last for 3 to 5 hours depending on the humidity. I toured in the south last year and was regularly riding in temps of 100 F with similar humidity. I packed my Camelbak each morning with as much ice as I could stuff in it and had ice cold water all day as well as a cold pack on my back cooling me down.

Considering that I live where the average annual humidity is 50% (which is considered muggy around here) and we regularly have humidities that are single digit, I'm not used to the humidity but the Camelbak let me endure it.