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).