Cruise lines do not hand search your carry-ons or bags. They x-ray them just like at the airports for contraband and weapons. Probably, those folks you saw were bringing on their personal beverages just like you may wish to do. Just don't consume a Bud from the can out in the open decks. This is tacky! Put your beer or soda in a cup, water bottle or enjoy your choice of beverages in your cabin. I am sure you will contribute to the "bar bill" on board ship as most of us do in lounges, at show, while dining, etc. Don't forget that tips are automatically included on bar bills, so it is not necessary to tip the bartender/waiter separately unless you want to. I alway bring one bottle of alchol in each carry-on and sometimes Sparkling Water (my regular beverage) which is hard to buy on board or very expensive ($2-$3 per bottle same for a can of soda). Regular water on board is fine to drink from the tap!
Bon voyage