Sail the White Sea! The Sky Knight and the Gate in the Clouds! - S6.E153

The Straw Hats arrive in an ocean above the clouds. Usopp almost falls out of the Going Merry but is saved at the last minute. The crew is also attacked by a mysterious looking warrior, but is saved by the Knight of the Sky. Finally, they arrive at the border which transports them to the upper realm, Skypiea.