We'll have another thing like with the Lethe River. You'll be dragged along a long oceanic current and fight a battle every now and then, having to choose a way to go on occasion.
At the first fork, go right to find an X-Potion . At the next fork, go right and into the cave. Go north to the next area, then south and onto the switch to lower the water. Jump down and go north to find a chest with the Green Beret within. Exit to the north and continue to Nikeah.