The phrase "part water" is an idiomatic expression that means to have some characteristics or qualities associated with water. Water is often used as a metaphor for various things, such as emotions (e.g., being "cold-blooded," like fish in water), physical properties (e.g., something being "waterproof"), and even people's behavior (e.g., someone who is very adaptable or flexible is said to be "like water"). So when you say that someone or something "part water", it means they have some of the characteristics associated with water, such as flexibility, adaptability, fluidity, etc.