The phrase "keep your head above water" implies that one is barely able to manage a difficult situation. There are several synonyms for this phrase that convey a similar meaning. For instance, "surviving" indicates that someone is able to make it through tough times. "Coping" suggests that someone is dealing with a situation in a practical way. "Enduring" implies that someone is able to withstand a difficult situation over time. "Hanging in there" conveys that someone is persevering through a difficult time. Overall, there are several ways to express the notion of staying afloat amidst challenging circumstances.