The Ho-Oh Ocean is the large sea to the east of the Mushroom Kingdom. Mario and Luigi had to dive to the bottom of the Ho-Oh Ocean (the Ho-Oh Ocean Seabed) during their adventure in Mario & Luigi: Sibling Rivalry.
This is the only stage with no warp pipe. It also connects to Yoshi Island, in the game Mario & Luigi: Rivals in Adventure. The Mario brothers must travel through here to get back to Yoshi Island to fetch the Wario Bros.