The wall would have been manned back in the day. Even if you sneaked a small force through, there's no way 10,000+ men are getting through, let alone wagons.
More than preventing invasions, it made it much easier to tax goods.
The thing was designed to keep armies out, not individuals. Plus there would obviously be guards at the end.
What a silly thought
Every time I see this I wonder how easy it would be for me to get around this. I’m sure it wouldn’t be a problem with a boogie board (depending on how strong the current is), but there must be border guards there to stop you. If you never set foot on land and go back to the other side is it an issue?
Easy enough to do physically, but slower than using a rope and climbing over. And you'd be crossing at one the most monitored spots.
No because assuming that wall is the border so many feet of water is still the territory of that country.
Canoes, shovels, and ladders.
The super-clever downfall of so many historical (and recent) walls.