jager heater query

could anyone with experience of the jäger aquarium heater enlighten me
as to whether "fully submersible" actually MEANS fully submersible? It
has a minimum water line and lots of lovely diagrams indicating how well
insulated the cable is, and an arrow pointing to a double sealing joint,
but no specific instructions as to a maximum level.
I'm assuming that I can in fact put the whole thing underwater (it's
been operating that way for a while now) but I've developed a sudden
paranoia about it. Can't find any definitive info on the net either, and
I thought I'd ask here before ringing the manufacturers...

many thanks for any help,
--
sophie