it's full of water so it has neutral bouyancy in the water -it shouldn't affect the life jacket's righting moment. Besides, most of the life jackets we use will not orient you face up anyway (try it). I always bleed the air out of my camelback after filling it so it doesn't slosh and disturb the beautiful wooshing wind and water sounds.

As far as the rules go - I can identify with it...I mean what's next? A jacket full of lead to give more righting moment? Granted, I think an exclusion should be made for camelbacks given that they fall below a certain weight or volume limit.