Thank you very much, everyone, for your replies! I appreciate that kind of practical and useful feedback, and you've helped me.
Next -- I feel a HUGE rant coming on!

The continual barrage of contradictions that I encounter in pursuing this wet hobby are enough to make me lose my mind.
HOWEVER, due to my high level of respect for the Planet Catfish site AND it's members, I will summon a high level of self-control, and suppress that rant.
Does this stuff happen to any of you guys? Is it just me? This is what I'm talkin' about:
While reading your responses, I checked out the link that was provided by
Mats. At that link, there was a page of information about the Jager heater, and a list of the heater's attributes. Among those, we read:
"The heater can be fully immersed..."
Okay. That's good news. But seeing that statement caused me to question my sanity. I was just about to call the local mental health facility, but then I had a moment of strength. In that moment, I said aloud, "No! I'm not imagining things! I KNOW what I saw yesterday, and many other times in the past! I shall get to the bottom of this!"
I then visited the Foster and Smith Aquatics website. They sell the Jager heaters. There, among the information for the SAME heater, we read the following statement:
"Submerse up to 'waterline' in vertical position."
Okay. That is consistent with what some of you guys pointed out, that the heater has to be IN the water; NOT sticking way OUT of the water. I think we can all agree upon that.
But just 2 inches below that, we also read:
Caution: "...Do not submerse beyond 'water line'."
Ah hah! It seemed as though I had verified the contradiction. In order to be certain, I visited the Merriam-Webster Online Dictionary, and looked up the following words: "submerse", "submerge", "immerse", and "emersed". According to Merrian-Webster, "submerse", "submerge", and "immerse" all mean, in simple terms, "to put down INTO the water".
So the instructions at the F&S website tell us to place the heater into the water, as far as the water line, but to NEVER put it into the water ANY FARTHER than the water line. DOAH!
IN CONCLUSION: I realize that an online seller could easily have an error on one of its pages, so I searched the web, checking out other sites where this heater is sold. Very quickly and easily, I found similar warnings, (telling us that we MUST NOT install this heater any deeper than the water line), on FOUR other sites.
So there you have it. Stop the madness!

Planet Catfish buyers beware!
p.s.
Mats -- I never thought about plecos trying to hide under the heaters. That sure sounds like something that they'd try to do. Good point.