The starting point is an image of a drowning person. If they keep their head above water (out of the water), they will live. If they don't, they will die. Figuratively, people can be drowning in debt and drowning in troubles and their goal is to "keep their head above water." So keeping one's head above water means just barely managing to survive (at work, at university, in life).

There are other meanings that would fit better in this context: - To just be able to continue doing what you do - To (barely) avoid being consumed by a stressful or unpleasant situation (often related to work or finance). - To manage to survive, especially financially The word "water" in this expression is used as a metaphor for troubles that can overwhelm you.

Her video clip just confused me =\ Thank you, Jazz!

With the idiom you mentioned, water is a metaphor for problems or difficulties. When one is trying to 'keep their head above water', they are trying to rise above their problem. In Lavigne's situation, her problem is the disease. If she doesn't overcome this problem, she will die (drown). People often use this idiom when they are talking about financial problems or responsibilities, but it can also be used to describe an emotional or turbulent time in their life (ie. health, breakups, loss of a job/loved one).