Question

why does heat travel faster upward in water than in downward direction on heating

Answer 1

because the hot air particles let move the water upwards

Answer 2

Heat is scientifically explained as movement and vibrations of the molecules and atoms of the water. Thus, when water is heated, the water molecules and atoms need more space, because they vibrate more. Since the same number of molecules of water need more space, they become lighter than the molecules of water which are cold,and more packed together. If you heat the water from top with a small immersion heater, you will notice that bottom water remains cold, it is heavier than the water on top. Even ice may form on top, but below in the ocean water remains as it is, because water at 4 deg. C. is heavier than ice. thus fishes can live below.