Hi b_javidsharif, welcome to PF! For homework-type questions (not that this is necessarily homework), we ask that you give your line of reasoning first, then you'll likely get useful comments.
Poisson's ratio can be calculated if you know the bulk modulus K and the shear modulus G of the material. In water, is one of these much greater than the other? What does this imply about the Poisson's ratio?