Which end of a water molecule is positive and which end is negative and why?

The region or the end of water molecule that is partially positive would be where the Hydrogens are. The end that is partially negative, is where the Oxygen is. These partial charges, are because of the large electronegativity differences present between the hydrogens and Oxygen in the molecule. Thus unequal electron distribution or sharing occurs between the atoms in the molecule causing it to be polar and possess the partial charges.