When water diffuses the process is called osmosis.
Water can cross the cell membrane through the lipid bi-layer and with special proteins.
If enough water enters the cell by osmosis, the cell can swell enough to burst open and die. This is demonstrated in the picture below. In this case too much water enters and the cell swells to the point of bursting open. In the end pieces of cell membrane are left in the water. Pieces of red blood cell membranes are known as red blood cell ghosts.