The highest percent oxygenation of the blood that can be achieved when blood and water flow are in the same direction is typically limited to around 90-95%. This is because as blood flows in the same direction as the water (usually in the gills of aquatic organisms), it has the opportunity to pick up oxygen from the water until it reaches a point where the oxygen concentration in the water and the blood reach equilibrium.
At this point, the blood becomes nearly saturated with oxygen, but it may not reach 100% oxygenation due to factors such as the solubility of oxygen in water and the efficiency of oxygen uptake by the blood. In practical terms, the actual percentage can vary depending on the specific conditions and the efficiency of the oxygen exchange process in the organism.