She had a lot of expenses and very little help, so she had to struggle just to keep her head above water.

keep 是动词，意思是"保持、维持"。 her 是代词，指代"她的"。 head 本义是"头部"，这里用作比喻。 above 是介词，表示"在...之上"。 water 指"水"，这里用作比喻。 keep one's head above water 是一个习语，字面意思是"让头部保持在水面之上"，比喻"勉强维持生计"或"避免陷入困境"。