The phrase "there must be something in the water" is a colloquial expression used to express surprise or bewilderment at a particular situation or phenomenon. It is often used to describe a sudden or unexpected change in behavior or circumstances, or to imply that there is an unseen or unknown factor at play.
It can also be used in a more literal sense to suggest that there is something physically or chemically present in the water that is causing a particular effect or behavior.
It can also be used to express a group of people or a community who are behaving in a similar way, for example "there must be something in the water, everyone is getting pregnant"
It's worth noting that the phrase is often used as a way to express surprise or bewilderment and should be used in a casual and informal setting.