I went thru half a qual once w/ my KEVLAR down over my eyes halfway.
Getting outta your pack can be tricky; especially if you are maintaining a handhold on your rifle. I'm with the opinion that my pack would be the first to go and I would do all I could to prevent from entering the water with it on.
eta and back to topic
Its called Survival strokes and you can swim with or without your pack strapped on.
(Theoretically) While swimming with your pack OFF, you place it in front of you w/ weakhand, and your rifle (forward) on top of the pack w/ stronghand. Basically like a scissor kick w/ your pack/rifle leading you.
You
can swim with your pack on, but it is alot less productive IMO. As per the above comment, I got hung up once trying to get outta my pack and drank a bunch of water, and then puked it back up