Amphibious aircraft pilot reported an engine failure shortly after takeoff and a subsequent forced water landing. After inspection; it was discovered that the turbo charger clamp had failed causing the exhaust pipe to come loose.

Narrative

After all preflight checks and engine run up of our Lake Amphibian aircraft; I departed for a local flight at XA00 from ZZZ airport; on Runway XX; and made a left downwind departure heading for a lake. I had one passenger who is a CFI.During the downwind climb the engine stopped; and I proceeded to make a normal power off water landing straight ahead close to the bridge of the highway on the lake and the lake boat ramp; 1.5 miles from the ZZZ airport.As we were paddling towards the ramp; a fisherman approached us and advised us that the engine was smoking; upon which I took a fire extinguisher and put the fire out under the fiberglass cover.The fisherman towed us to the boat ramp; and we proceeded with the help of the aircraft maintenance shop truck to tow the airplane out of the water on its own landing gear to a flat area next to the highway.Upon initial inspection by our mechanics; it was determined that the turbo charger clamp holding the exhaust pipe had failed as the exhaust pipe was loose causing the engine to stop.The aircraft and wings were mounted on a special trailer and trailered to the maintenance shop.The delay in filing this report was caused by our insurance company request that they be present when the engine cowling was removed; and a detailed inspection could be carried out. This request was later waved; and the engine cowling was removed; and a detailed inspection of the engine failure was carried out at the maintenance shop; confirming that the engine stopped due to hot gases on the magneto caused it to fail during the initial flight; and a leaking oil breather rubber hose that also melted with the turbo exhaust and fiberglass cover caught on fire from the remaining heat of the turbo charger after we landed and came to a standstill without forced air going thru the cowling.After further investigation from our mechanics; we recommend to prevent a recurrence and correct this situation that the FAA proposal of an AD be published.