Indeed it was flimsy. But that was the price of making it amphibious. Against troops armed with light machine guns and rifles only it could be very dangerous. But give the troops a .50 cal or almost any heavier weapon and the PT-76 could be in deep trouble.
I recall now pix of a PT-76 that Ma Deuce turned into a collander. It was half submerged after several .50 hits made it go glub-glub-glub-glub.