Just be careful not to wear a camel back when you're racing as you will probably end up with some a**hole protesting you! Perhaps it makes a difference to the very very top sailors but the difference is so slight that you would gain/loose much more by having the sails slightly out of trim or by steering a couple of degrees off line than by the extra 1kg on you're back rather than on the tramp. Not a rule that I agree with.

They're a great idea for recreational sailing though.