On a Cruise ship, most have adopted a new policy over the last few years in regards to the tipping/gratuities. Most of the cruise lines, including Norwegian will add $12.00 per guest, per day to your on-board account. Though it can be more a little bit more, depending on the specific Cruise Line you are sailing on.
They do this for several reasons. Legally you can not prepay for tips, so all the cruise lines add it to your account each day and then charge it to the credit card you provided to the check in attendants on embarkation day when on the pier, on the final day. This way you are not tipping people with money, while on the ship and you are not handing anyone a money filled envelope at the end of the cruise for tips, unless you want to.
They always have envelope for tips down at the customer desk, so that you do have the option of providing an additional tip to someone you think really deserves it for outstanding service. You also have the option of asking them to remove the money they are adding for the tips, or to increase it, by going to the customer service desk. I recommend making sure you do this before the 6th day, as things normally tend to get busy at that time when people are starting to settle their accounts. This means you can avoid the lines this way.
When we travel with our teenaged sons, we ALWAYS make sure their room attendant is compensated for the extra work they do (boys!) because they have always had to take away hundreds of room service plates, pick up wet towels from all over and clean up the mess of their clothes, so we always make sure to catch them in the hallways and give them an envelope on the 2nd last day or so.
At the conclusion of the voyage, the ship divides the tips they have collected between the serves and attendants in a pre-determined order according to a schedule of percentages. For instance, the head waiter will get more than the assistant waiter, who will get more than the table waiters.
In regards to purchasing drinks, when you are asked to sign a receipt for the charge to go to your room, make sure to take a look at it before you sign. In most circumstances, there will be a tip included down at the bottom, even though they leave a line open, asking for a tip amount. It looks a bit misleading because it seems like you have to add a tip that they have already placed on the card.
The only place that I have experienced where they do not include a tip for rendered SERVICES (as opposed to purchases), is inside of the Spa. There people normally add 15-18% dependent on what they are having done. A masseur may get more of a tip than someone who changed a nail polish color.
When in destination, I normally tip at the regular rate of 10-15% for taxi rides or for serving staff at restaurants we may be eating or drinking at. We always make sure we have a lot of single Dollar Bills for this as in the Caribbean, they are very happy to accept US currency.