People have often tried to hide drugs and alcohol in water bottles. Even if they'r legal items, they may not be permitted in certain places. For instance, a 21-year-old college student could try to sneak a drink into a football game on a dry campus.
So, if you're stopped by the police, are they allowed to inspect your water bottle?
The answer depends on a lot of factors. For one thing, do they have reasonable suspicion that a crime has happened? If so, they may be able to conduct the search. For example, if you were pulled over on suspicion of drunk driving and blew a 0.09 in the breath test, they may well be suspicious of a water bottle in the passenger seat.
Where you are and how much privacy you expect also comes into play. You have the most privacy in your home and can often object to any search of the house without a warrant. You don't get as much privacy if you're in the car or out in public.
This may seem obvious, but it also depends on whether you gave consent or not. If an officer thinks you're drinking in public, his or her first step will likely just be to ask to check your container. If you allow it, whether the officer should have been suspicious or had probable cause or not, the search can continue. Some people will refuse all searches, even when there is nothing illegal in the water bottle, just so that any further search violates their rights and potentially invalidates any evidence.
When talking to the police, it's very important to know your rights. The same is true after a search and an arrest.
Source: FindLaw, "Can Police Search My Water Bottle?," Christopher Coble, accessed Nov. 23, 2017