The fact is we don't even know how much percent of the users body has to be under water for it to work.
If the GM ate a DF then it would become exactly like the boat you get in the Wind Waker. A talking boat who's head piece can move and talk to you and since only part of it's body is underwater, it could kinda wade with great effort.
Since we already know of GM spirit, I'd say she didn't eat a Df out of nowhere.
If we knew the SH had a Df on their boat it'd be the other way arounf, but they didn't.
Maybe we should just wait for the explanation, which has to come in the next chapters.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"