This mission is similar to "Through the Jet Stream" in Jolly Roger Bay. We swim through the channel to the other area. Near the seal, underwater is a grate with rings and a jet stream coming from it. After floating (or standing) through 5 rings, each with its own number, we hear a chime and the Star appears over the grate. The rings will continue to appear. Because of the jet stream, we need the Metal Cap to reach it. Run quickly, as the power does not last very long.
Star Get #70, 4 Coins, 4 Lives
We get a message: "You've found 70 Power Stars! The mystery of the endless stairs is solved, thanks to you -- and is Bowser ever upset! Now, onto the final bout!"
Well, we can now tackle the final Bowser with 70 Stars, but we can't reach him until we beat him a second time. We haven't even discover these mysterious stairs, and we've already taken care of the problem! This is what happens when you try to be thorough.
We need Wario for this mission. Wario is a slow swimmer, so watch your health meter if you start the course with him. We go through the tunnel and get the Power Flower. With the Metal Cap, we fall to the grate and stand through five rings and jump up to get the Star when it appears.
Star Get #87, 6 Coins, 3 Lives
The next Star is Koopa Surfin' Switch Star.