Unreachable build near Pitcher

Is unreachable because your dupe cant breath under water long enough. Better build some leaders under the existing ones for going into the building dupes area. You can solve also whit a dupe equipped whit atmo suits. 

Thanks for you comment, but I don't think that's the case.
After I deconstruct the first insulated tile ( left -> right ) at the bottom, it became reachable.

It was only considered reachable because one dupe was in the water and close to the build designations. Or because the water dipped.