If you have any issues with your shoulders, you may well find a back-pack hurts.
A large back-pack will need to go in the baggage hold of the plane, and to avoid damage to the straps and click-fasteners, you will need to either sew or find a suitable cloth bag or sack to put it into while in transit.
A small back-pack can go as hand luggage, but check with the airline about what size they allow as cabin luggage. Most airlines do not permit you to have any liquids in your hand-baggage these days, except medicine that you need in -flight and have a doctor's letter about, so the water bottle will need to be empty.
I have never used a cross body bag, so can't advise on them.
Never have money, your passport etc, in either hand-luggage or other luggage while travelling. Keep it in a money belt, strapped round your waist and preferably tucked into your underpants as well. That way you will assuredly notice anyone attempting to steal it. I pin my money belt with safety pins to my clothing as well for extra security.