Running brushless motors under water is no problem. Check this vid and read the comments on the vids Youtube site:
http://www.youtube.com/watch?v=rdlsQhqXlw8
However it's not that simple....obviously some parts of the motors (bearings, contacts, etc...) could be affected in the longer run. Most of it is covered in the Youtube Comments section of that vid.
I guess the idea is not to dump the copter on porpose all the time under water, but I see it more as a sleek safety feature if you fly a lot over water.
When we were in our RC-car madness, we used to make the motors and ESC's waterproof (mainly Crawlers and Monster Trucks) in order to drive in snow or rain puddles, and never had a problem. You could also make the motors from this copter waterproof (some people even dunk parts of the motor in Epoxy and machine it down...) , take ceramic bearings and use a lot of liquid electrical tape, the question is just if it's worth the efford.
Chris