Sink or swim time is an expression that is often used to describe a situation where one has to succeed or fail completely on their own. When it comes to finding synonyms for this expression, there are several options that convey the same idea. For instance, the phrase "do or die" is an idiom which conveys the same sense of combative determination and survival. Another synonym of the phrase "sink or swim time" is "make or break moment." It stresses the importance of the situation, where the outcome will either make or break the person. Other similar expressions include "all or nothing," "now or never," and "put up or shut up" among others.