OK, I wasn't thinking right,
The place with *** is the center of the point on the zoomed out map, i.e. the middle of a city or the middle of a square of plains, etc.
Like others said, you have to walk around partially zoomed out til you see sewer access in the buildings list.
It'll be a 9 square area that you can find this, and you just go to the middle square and look around behind a building for a stairway.
I taught myself to swim and tried going through that drain earlier but it was wall to wall water so I drowned again.