Is my Tetra submersible heater supposed to have a portion of the cable in the water?

It is an HT10 50W Submersible Heater by Tetra. The instructions don't say that the cable is supposed to be in the water, but it shows a picture of the heater suctioned to the side of the aquarium with the cable in the water and plugged in... this just doesn't seem right. What should i do?