When you kill the great shark, head back through the piranha cave. Head across the next
pool, moving through the narrow flow to turtle bay. Stick to the right wall, passing a
tunnel on your right. Ahead you'll see a tunnel partially blocked by rocks. Swim over them
into the small tunnel beyond. Snag the Power of Vigor near the glowing spot here. Then
fight the current, repeatedly pressing the Charge button, until you reach the glowing
point.