I use one of these a lot of the time: http://www.amazon.com/SPEG-UV-Pro-Cy.../dp/B00AWU9AZO

I won't lie and say it's cooler than wearing nothing, but it's the thinnest one I've found and if you wear it all the time you'll get used to it. It does have a cooling effect when it's wet, but that doesn't last too long here in Nevada. YMMV in Florida.