I was at a float fly once and a guy was flying an Aerostar 40 with a set of blow molded (hollow) floats. He had a hard landing and later was taking forever to take off (we realized later that the float had split and was filling up with water). He finally lumbered it into the air and after a few circuits the wing folded from the extra stress caused by the weight of the water in the floats. The engine sunk to the bottom of the lake (he didn't secure it with a wire fishing leader like I do). May be this is a one only case, but I wouldn't use blow molded floats.