If you're talking about a waterway:
* Natural waterway: This refers to a river, stream, or other naturally occurring watercourse.
* Ocean: A vast body of saltwater, which is the opposite of a man-made, controlled waterway.
* Land: This represents the opposite of water, signifying the absence of a canal.
If you're talking about the act of creating a canal:
* Natural: This refers to something existing without human intervention, the opposite of a man-made canal.
* Destroy: This means to demolish or break down, which is the opposite of building a canal.
If you're talking about a canal as a means of transportation:
* Road: This represents a terrestrial path for vehicles, contrasting with a water-based canal.
* Railroad: This refers to a system of railway tracks for trains, providing a land-based alternative to a canal.
Ultimately, the best "antonym" for "canal" depends on the specific context of your sentence.