From the Super Mario Wiki
Magma-River Cruise, or Peach's Castle-2, is the second course of Peach's Castle in New Super Mario Bros. U. It takes place on a raft similar to the one found in World 5-4 of New Super Mario Bros. Wii.
The course takes place on a raft, and the raft itself can only hold up to five enemies/objects aside from Mario and other playable characters. Monty Moles and Mini Goombas make a prominent appearance in this course, as well as ash.
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