Drop your weapons or put them in your pack (this will not work if you have any weapons equiped)
Start running (W) with or without sprinting (shift)
Press and hold left control
Run into the ocean
Continue running

Video: http://youtu.be/iB9leWklyiw

Is left control hard coded as some kind of turbo key? I don't see anything special bound that key in settings or in the arma3profile file and this works for everyone by default.

Secondary bug:
You need to continue pressing left control and running after you resurface until the "drowning blur" has faded. If you release control too soon you will die but if you wait long enough you live. Shown in the video during the second example.