Yes, you are absolutely allowed and even encouraged to bring your own tumbler cup, such as a Yeti, Stanley, or Tervis, on a Carnival cruise. Bringing a reusable, insulated tumbler is a "pro-tip" among frequent cruisers for several reasons. First, the standard cups provided at the Lido deck buffet and beverage stations are quite small, so having a large tumbler allows you to stay hydrated with fewer trips to the water or lemonade dispenser. Second, the insulation is essential for keeping your drinks cold while lounging in the intense sun on the Serenity deck or by the pool. To comply with health and safety regulations, you are asked not to place your personal tumbler directly under the beverage dispenser nozzle; instead, you should use a clean, provided plastic cup to fill your tumbler. This prevents the spread of germs. Furthermore, if you have a drink package like "Cheers!", many bartenders will gladly pour your cocktail or soda into your tumbler if you ask politely, ensuring your "Miami Vice" or "Fun Ship Special" stays frozen for much longer. Just ensure your cup is empty when you go through terminal security at embarkation, and avoid bringing any glass tumblers as glass is generally prohibited in pool areas for safety.