The phrase "at the mercy of wind and wave" signifies being completely vulnerable or powerless over uncontrollable forces. There are several synonyms for this phrase, including 'at the mercy of fate', 'at the mercy of circumstances', 'helpless', 'vulnerable', and 'exposed'. These words perfectly capture the feeling of being forced to surrender to external factors outside one's control. Additionally, 'subject to the caprices of nature', 'dependent on external forces', and 'enslaved by fate' all convey the idea of being powerless and helpless in the face of unpredictable or inevitable events. Whether it's navigating through the rough seas of life or coping with unforeseen circumstances, these synonyms capture the essence of the phrase 'at the mercy of wind and wave'.