These phrases "melted" and "became like water" share similar meanings and are combined to emphasize that the people were extremely afraid. (See: [[rc://en/ta/man/jit/figs-parallelism]])
Here the people are represented by their "hearts" to emphasize their emotions. Alternate translation: "The people were very afraid" (See: [[rc://en/ta/man/jit/figs-synecdoche]])