OMG THIS IS HORRIBLE!!
I love making a straight grid (probably compensation, IYKWIM).
And it is so impossible sometimes. The actual grid option, does not work and snapping has weird multiple ways to snap at the same angle + length and not be perfectly snapped with all of them.
You might have to remake some of your roads, or even start from scratch, but try the following settings:
Once you terraform water away, so that you can build there. The game does not always register that the water is gone and keeps thinking you are not allowed to build there. It does, however, allow you to build water drains there, since it is "meant to be placed in water".
Optionally you can add videos from Youtube or Vimeo to your tutorial