If you say you're trying to keep your head above water, you mean you are just managing to survive despite not having much money.

形容某人生活過得去，但手頭緊無餘錢，勉強可度日！

水除了一般我們常見的意思外，它也當成水面、水位、水域來看，可以衍生成一種境況，所以＂above water＂指擺脫（經濟）困境，而＂under water＂就指生活（經濟）潦倒。

water前面加上了troubled、muddy、deep就跟困境麻煩等扯上關聯，如成語＂fish in troubled waters＂混水摸魚，趁火打劫；前面加上了smooth，就成了順境，如片語＂get to smooth water＂進入順境，也就是安然擺脱困境了。

Phrase from BBC learning English