i always use the garment bag, but if you pack it in the plastic cleaners bag and put it in last it should be less wrinkled.
There is no need to pack snacks, the ship provides lots and lots of food.
If you must "smuggle" in soda then i would pack bottles in my checked luggage. Buy it in port before you sail, wrap it very well, you wouldn't want sticky soda all over your clothes.
Don't bother to pack water, the ships tap water is very good. Go to the grocery store in port before you board the ship and buy a couple of small bottles of water, carry them on with you and re-fill those bottles from the tap