Curious to see how Cruise lines handle empty water bottles / thermos?

I've mentioned before, since we have to fly anywhere there is a cruise ship, we don't take much. But we do buy thermal cups and water bottles on board the first stop we make at the store on board. We get morning coffee and water throughout the day . It's easier than bringing something on board.

Usually, if they are empty, there is no problem. We had a coozie cup from the stay the night before and brought it on with no issues. People do it all the time.