Cities: Skylines only fully renders the unlocked tiles. Locked (not yet purchased) tiles and all tiles in the editor are not fully rendered to save processor power. When you create a tunnel entrance or a canal, it is buried under earth.
Subscribe to the “Common Ground” mod to fully render any number of tiles you specify. The Options page of the mode allows you to select what to render in the game and in the Editor. If you use the 81 Tiles mod in the game, you can disable the Common Ground mod, but you still need it in the editor, as we need to disable the 81 Tiles mod while using the Editor.