The expression to ‘keep your head above water' is mainly used when speaking financially, although it can also mean to survive or be surviving terrible circumstances of hardship in one’s life. Financially, it means to find oneself in a difficult financial situation and are barely managing to survive. Also, when someone is finding something overwhelming, they might be finding it hard to keep their head above water. This is a popular expression in the English language that is more common to hear in a formal setting.