I’m not an active ‘cruiser’ but I can answer most of your questions.
All port charges and fees should be included in your bill prior to paying. Most internet companies show everything up front but a few have ‘fees’ that are tacked on just prior to billing. There should be no extra charges EXCEPT for gratuities unless you order drinks or have a shore excursion, etc.
Gratuities are almost always added on to your bill and not at your discretion. You can challenge the quality of your service and get a reduction, but that may take hours of waiting in line. Each line will have gratuity charges stipulated on their web site and should be considered mandatory - IMHO, just because it’s not easy to change.
Drinks are much more in the $10. range but some cruises will have a duty free store that sells booze (so I’ve been told) or your best bet is to smuggle a bottle on when you first board. Soft drinks are extra too. Most cruises will have soft drink cards that allow unlimited soft drinks for the cruise. A few are offering the same thing for wine, beer and some bar drinks. Very pricey though and only good if you drink a lot. Best bet is to head to a bar on a shore cruise and stock up - in your stomach.
Entertainment? I’ve seen some pretty good and bad stuff both on AI’s and cruises. Just depends what you want. On both our cruises, we found quiet piano bars that suited our tastes, something that is not easy to find at an AI.
Buffets are almost always far superior to AI’s, HOWEVER, most cruises now charge a fee for the upgraded dining areas, such as steak houses or Japanese. This is often $35 to $40 pp.
There are lots of sites dedicated to cruises. Best to check there. One i found that might be useful if you haven’t booked is Cruise Compete. After you sign-up, you post the cruise & date you are wanting and a dozen cruise agents post their best prices on the site for you to compare. None get your home email so it works well.
Bottom line to me is that a cruise is not as relaxing as a week at an AI. You always seem to be on the go. However, I just returned from 18 days in Italy and it was the most rewarding and least relaxing trip we’ve ever had. If you can combine a cruise with a week on the beach after, it’s a great deal.