What causes oxygen from the water to diffuse into blood?

The diffusion of oxygen from the water into the blood is driven by the difference in oxygen concentration between the two environments. Oxygen concentration is higher in the water than in the blood, so oxygen molecules will naturally move from an area of higher concentration to an area of lower concentration. This process is facilitated by the presence of specialized structures in the gills of aquatic animals, such as lamellae, which increase the surface area available for gas exchange.