Well, living down here in Texas I've done a few shows in 100 degree-plus heat and yes - lots of cold water is NECESSARY.

Something you might consider doing - if you have a small portable cooler go get a bag of ice, put it in there and then when you get to the gig open the lid so the ice will slowly melt; be sure to bring along a cup or mug to throw into the cooler so you can drink the ice-cold water that comes about as a result of the melting ice. It should take several hours to totally melt so you'll have plenty to drink. You can always add some water at first to sort of "prime the pump," if you will.

Also, I don't know what gauge strings you use but "light gauge" will not work for me in the heat - they get corroded very quickly.
It may well be because of the sweating and the resulting excess acid from my skin but they do corrode quickly and just sound terrible and then have a tendency to break. I always make sure my instrument has medium gauge strings on it when I'm playing an outdoor gig in the heat. Also, be sure to bring a couple of towels - one for yourself and one for your instrument and keep it wiped off between sets.

A hat is usually a good thing to bring along, too; or, at least a pair of sunglasses in case you are uncovered or the sun-in-your-eyes becomes a problem. Finally, the host should provide water for the
entertainment, but either way, I'd bring along that ice chest with some ice in it. I've done some hot Texas gigs where they provided the water but it was warm - yuck. Cheers.