The new coastline system leaves a canal when you use the landfill.
In [0.15.40] it looked like:
0.15.40_before_placing.jpg 0.15.40_after_placing.jpg
In [0.16.6] it looks like this:
0.16.6_before_placing.jpg 0.16.6_after_placing.jpg
Now the problem is, that there is a canal which is walkable, but ...