A boy carries a fish in one hand and a bucket of water in the other hand; if he places the fish in the bucket, the weight now carried by him:

The water in the bucket applies some magnitude of buoyant force on the fish, that is exactly equivalent and is opposite to the force applied by the fish on water. Due to this, both forces cancel out their effect, resulting in no variation in the weight carried by the boy.Therefore, the weight carried by the boy will remain same as before