I placed industrial sink on top of a counter, placed a round water collector barrel thingy directly above the sink.
I didn't get an option to plumb them together.
Tried right clicking both of them,
Tried placing barrel on top of crafted floor tile,
Tried placing barrel one tile above and one tile behind the sink,
Tried placing barrel one tile above and one tile to the side of the sink,
Tried placing barrel one tile above, one tile behind and one tile to the side the sink,
Tried placing barrel first and sink second, and vice versa.
Nothing gives me the option to plumb.