Go to just past the "K" until the water rises, then swim back to where you fell down at. You will see a banana coin and a banana bunch. Directly to the right is a space. Swim into it, and it will take you to the end of the level. At the end, you can use your partner and jump high to the left and throw him/her straight up and to the left to get your DK coin.

Proceed as usual at the start of the level to get the water to rise. Then, back track to the beginning where you will see a small pocket opening, about two crates long, at the ceiling. It is recognizable as an area where there would be a K or a Banana Coin, but there is nothing there. Proceed and you will get in an invisible barrel. This will launch you to a small room with banana-shaped exclamation marks. From there you will go to the end of the level.

Wait until the water goes up and go back into that opening where it appears like a barrel will fit.