A flying boat uses it’s hull for primary flotation, has no retractable landing gear and can take off and land only on the water. It has no pontoons other than possible outriggers on the wings for stabilization It comes ashore when beaching gear is attached to the fuselage.
Amphibians, on the other hand can take off and land on the water using the hull or pontoons for floatation or retractable landing gear for takeoff and landing on land.