"immersed" and "submerged" are synonymous (under water), and "emmersed" is the opposite (above water)..hope this helps..
by the way, i can't find the word "emmersed" in the dictionary..but if you search among planted tank forums (international), you can see the word "emmersed"..in fact, i'm starting to believe that "emmersed" is just an aquarist's lingo..