Wario Land 3: Part 71: The Tidal Coast (Blue Chest)

The Octohon's are now huge in this level, thanks to that Sapling of Growth. Head to the doorway at 0:20 and you'll find them in the next room, which shows that Wario can now swim through more walls. Find the Blue Key somewhere further down in this room, and then head to the doorway at the bottom to reach the Blue Chest. The Tidal Coast's Blue Chest Treasure is a Green Gem, something else for Wario's greedy hands to keep.