I knew this would come up...
What I understand is that when Eheim aquired Ebo, they had to go through some recertification processes all over again, to get the heaters certified for submersible use. The heaters are the exact same units they used to list as submersible, but until they get recertified, they couldn't call them such anymore, so they had list them as non-submersible.
If you call any of the major vendors or the manufacturer, they will tell you they are in fact submersible, the instructions, specs, and heater casing contradicts that and will only until they get recertified.
Now I'm certainly not going to tell you whether or not to submerse them, but Marine Depot, Foster and Smith, and Eheim themselves have told me that the heaters are submersible, despite the directions. I have submersed mine with no problems, and numerous other people here have too, but make your own decision as to what you want to do. I would suggest calling the manufacturer and hearing it from the horse's mouth yourself.
Edit: I just visited Eheim's web site and they now list the Jager as submersible again, so they must have gotten re-certified!
http://www.eheim.com/jager.htm