When it's really hot and humid but not sunny I make sure to have plenty of fluids - a cooler full of water bottles, 'cause I'll fish 6-8 hours straight, plus my trusty bandana.

If hot & humid and sunny, I wear my Tilley hat, long sleeve +30PF shirt and full length ultralight pants and sun gloves (tips are cut off) + bandana around my neck. I can dip the bandana in the lake and re-tie it around my neck anytime and squish water out of it onto my legs  / arms / back as well.

I fish from a boat and catch a lot of my bigger fish between 11am and 3 pm.  I look overdressed, but I'm actually cooler than many people with no shirts on (specially when sunny). Dan