There are many synonyms for the phrase "plunge in water" that can add variety and nuance to your writing. Some of these synonyms include "dive," "submerge," "immerse," "sink," "drop," "splash," "fall," and "plunk." Each of these words has its own connotations, so be sure to choose the one that best fits the tone and context of your writing. For example, "dive" might suggest a deliberate, athletic action, while "splash" might suggest a playful, carefree attitude. Experiment with these synonyms to enhance your writing and keep things fresh and interesting.