The Fluval E is fully submersible. I experienced the same concerns when reading their instructions until I figured it out:
The pictures that are "don't do" -- are saying that the heater must be mounted.
- One of the pictures shows the heater just hanging by the power cord, which is bad.
- The other picture shows it laying on the bottom of the tank, which is bad.
As long as the Heater is completely underwater, and it's suctioned to a wall, you should be golden.
"To know that we know what we know, and that we do not know what we do not know, that is true knowledge." -Confucius