We always bring our own insulated tumblers aboard when we cruise for exactly this reason! It's a great way to both reduce waste and have a larger cup than what is provided at the beverage centers!
Had no problem bringing aboard contigo or stainless steel bottles on board. While they were empty on embarkation, they had water in them when we returned from shore excursions and no one had any issues.
And you'll be happy to know Guests can bring water bottles or insulated cups to use while onboard. They are great to have to keep you hydrated during your vacation.
Many cruise lines let you bring water onto a cruise with you, but not all do. Some block it completely except for if you need distilled water for medicinal purposes, while some others won't let you bring single-use plastic bottles onboard, but permit cans and cartons which can be recycled.
Your all-inclusive resort will serve you drinks, but they will most likely come in small plastic cups that leave you heading to the bar for refills all day long. By bringing a Yeti along for your getaway, you'll keep your drinks icy cold while having to ask for fewer refills overall.
Carnival Cruise Line: Carnival has one of the loosest beverage policies of all cruise lines, allowing each guest to carry on a maximum of 12 sealed 12 oz.cans or cartons of a non-alcoholic beverage (e.g., sodas, juices, milk, or sparkling water) – a policy that most cruise lines have eliminated.
You might be a big fan of bottled water and refuse to drink tap water, but a cruise ship's water is MUCH better than tap water. They have high-tech filtration systems that allow the water coming out of your bathroom sink to be clean and more than pure enough to drink.
Water is always free of charge on most cruise ships. You may be asked to pay a fee for some versions of bottled water. However, you can likely ask for a jug of iced water in any restaurant or bar for no added cost. There are several water dispensers present on a cruise ship so that you can help yourself.