Not sure what yours looks like but mine too has a water line level. You are NOT suppose to put in any deeper than the water line (at least in my case) because you could "in theory" have an electrical fire in the fish tank. Just thnk about the wire having a cut in it and the heater being all the way in the water. Won't be good for the fish, the tank, or your place..
I have a submersible heater with a water level line too. Its if you want to have the dial sticking out of the water, it shows you how much can be exposed to air.