When you purchase a drink, they give you a receipt to sign - a 15% gratuity is automatically added to the purchase. There's a line on it where you can put an additional gratuity if you feel the bartender/server deserves it or if you feel it'll get you a bit more/better service. And yes, if you find the right bartender and use them regularly, an extra gratuity is appreciated and may be reciprocated in kind.
As for the luggage tags, while you can put them on before going to the airport, it's best to wait until you're actually going to the pier since they are usually not attached very well. If they do get misplaced, the airline will work with you to get them delivered to the ship, even if they have to go to another port. Just make sure you have some really good name tags attached to your luggage. And it never hurts to put an itinerary inside the suitcases just in case.
As for your back surgery, we hope everything goes well and you're finally able to get rid of your pain. I know how bad that can be, so our hopes and prayers are with you.