Simple, dig a canal and flood it... Try and dig under it then.
Cost to build wall at current estimate is about $25 billions. 30 feet tall & 6 feet deep. If you add a canal means......
1. You have to make the wall deeper far more than 6 feet deep.
2. Making a canal that long and deep is an enormous task and cost by itself.
3. Plus a water system ( aqueduct) so it will not dried up. Where is the water system coming from?
So the $25 billions becomes what? $30 $35 $40 billions??
Petulant Trump having a very hard time getting $5 billions after 2 years. Let alone going way over $30 billions???
Donāt worry bud...... they will not build a canal for your wall.
Also to build something like a wall or a bridge that is purposely to go on a water. Are entirely different kind of construction and materials.
You and I can build a wall. But you and I cannot build a wall that are designed to submerge to a water.