Imagine the hilarity if the giant bathtub train car derails and tips over. Has there ever been a train wreck that caused a flood? If not, there's a first time for everything.

There's also the possibility that you get 100 km inland and then the bathtub springs a leak. Not a good situation if the ship's structural integrity depends on the water in the tub.

Or - hear me out here - we build a giant man made ditch. Maybe we call it something clever, like a canal?

Reminds me of the Falkirk Wheel https://en.wikipedia.org/wiki/Falkirk_Wheel

Or the huge boat lift of Strépy-Thieu: https://en.wikipedia.org/wiki/Str%C3%A9py-Thieu_boat_lift

If it’s stupid and it works… !