I have a fort with a perpetual waterfall and I have a drainage point to add. Make your drainage tunnel 2 tiles high with no floors.
I channeled up to the surface brook so I would pierce it from the side. Then I led the tunnel to just above the only hallway separating my workshops and stockpiles. So I have this fort split completely in two.
The hall is 9 long x 3 wide.
This hallway is dug down with ramps 7 tiles apart, so there's floor - ramp down - 9 empty tiles - ramp down - floor. Across the hall against the walls are two rows of floor grates for the dwarves to walk on.
The next level down I have another set of ramps down, this time 5 empty tiles apart. Another pair of grates for travel.
The second level down is the ramps up 5 tiles apart.
Now I have a three-wide tunnel leading off to the map edge for drainage. But the important part is that my drainage tunnel is 2 tiles high with no floor in between.
I build fortifications behind the floodgate at the brook. Then I build another floodgate above the hall to stop flow there. The water will fall down onto the floor several levels below, sending up mist to the main hallway. Then I have fortifications at the 3 wide by 2 high drainage tunnel to stop people from being dragged down the tunnel if they fall in. This drainage tunnel quickly fills to a level of 7/7 but then excess water flows down the tunnel rather than backing up into the main mist hallway. It exits at a 5-wide carved fortification at the map edge.
Also, at Z-1 from the main hallway I have a fishing spot in a 3x3 shaft going from my well above down to the bottom level. It's on the other side of the hall from the drainage tunnel.
Works perfectly. Just lags. Also even though I have a constantly flowing water source the fish run out quickly since it seems to be based on area rather than water flow.
Side note: I always leave an empty level between floor of my fort. So as you scan downward it looks like FORT - nothing but stairs - FORT - nothing but stairs - FORT etc. This way I have left plenty of room for future plumbing at the cost of one extra tile of walking from floor to floor.
Dividing my fort in half forces everyone to experience the joys of falling water, but it also keeps my bedrooms separate from work going on elsewhere. There's a stockpile buffer and then a row of nobles' offices to keep the stockpile sound from waking dwarves up. Of course the bedroom is a mass-flophouse affair, full of master quality beds and everyone sharing a bunch of cabinets. Artifact furniture strewn in a line down the middle so everyone can slobber all over them. Same with the dining hall.