After losing his main client, he took on several part-time jobs just to keep his head above water.

keep 的意思是"保持、维持"。 his 是代词 he 的所有格形式，意思是"他的"。 head 的本义是"头部"，在这里比喻为"生存状态"。 above 的意思是"在...之上"。 water 的本义是"水"，在这里比喻为"困境"。 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"，比喻为"勉强维持生计"或"避免陷入财务困境"。