Hey,
Drowning i find to be a hard one to swollow.
As waterdragons can hold there breath for up to 2 hours!
from the day they hatch they hit the water and sink to the bottom as i breed 60 last year.
i am trying to help could you give a few details of the set up/enclosure as we might be able to find something else relating to the death.
As mentioned these little fellows also play dead.
I have had them sitting on logs then roll of playing dead i have picke them up walk from 1 end of the house to the other to show my wife when i hold it out to show her it jumps up and runs away.
if kept indoors need UV nad heating, water should be enough so that they can submerge there whole body and swimm if need be.
Any more info would help.
Darin