"Unstable as water" is a phrase that describes something that is constantly changing or unpredictable. Synonyms for this phrase include "volatile," "fluctuating," "capricious," "erratic," "unpredictable," "inconstant," and "fickle." Volatile refers to something that is susceptible to sudden and unpredictable changes, while fluctuating describes something that is constantly changing in a cyclic manner. Capricious and erratic are similar in that they both describe something that is unpredictable and impulsive. Unpredictable and inconstant focus more on the idea that something cannot be relied upon because it is always changing. Fickle, on the other hand, specifically relates to someone who has a tendency to change their mind or feelings frequently.