What is the origin of the idiom "be in over one's head" and when to use it?

The idiom "be in over one's head" is related to water and swimming. When someone is in over their head in a pool or body of water, it means they are in a situation that is deeper than their height, and they may struggle to keep afloat or maintain control. Figuratively, this expression is used to describe a situation where someone is facing challenges, responsibilities, or circumstances that are beyond their ability to handle or understand.