Yes, you need to understand refraction, because refraction is when an object bends when going into a different medium, meaning, when you put the spear from air to water, it will bend the shape making it harder to spear the fish. Hopefully that helps!
You will need to understand refraction, because objects you see from the surface of water, are not really in the same place as you see them. for example, a fish that you see in a pond might be slightly to the left or right underwater.
