Hi Mel,
Its a simple enough thing to do. You would be forgiven for thinking that submerging the tree like that would be enough to kill them, but for the reasons I explained above... they wont drown without detergent.
Soak the tree for a few hours, 3-4 should be sufficient, then remove and rinse. Leave it for a few days, then repeat the process if the mites are still there. Carry on this regime till the mites have gone.
I'm a big fan of this method, as its worked successfully for me in the past, and also doesn't harm anything, other than the bugs, plus you get nice clean leaves as a result!
Good luck and happy drowning
All the best,
Aaron