Here is a link to Florida's Glass Bottom Boats at Silver Springs.
The glass wasn't tempered, just thick. I think the thickness would depend on the size of the boat.
Lexan would be my recommendation in any modern build. http://www.silversprings.com/cruises.html
"Silver Springs’ popularity flourished after Hullam Jones invented the glass bottom boat there in 1878. By installing a glass viewing box on the flat bottom of a dugout canoe, he created a window to an underwater world teeming with fish, turtles, crustaceans and fossils more than 10,000 years old." http://fcit.usf.edu/florida/photos/r...lv/silv104.htm