Yeah, it's a bit hard to visualize, and the wiki is confusing about it (0__0 is correct, though). The article about retracting bridges uses different words:
Dig out ramps leading up to the first level below an ocean. Build a retracting bridge on that level, directly over the ramps (be sure to leave them in place!) and link it to a trigger. Carefully seal off the chamber to make it water tight. Now with the bridge in place, designate ramps up to the ocean adjacent to the bridge. Diggers with access to the level below the bridge can dig those ramps up from the level of the bridge, allowing the ocean to fill the chamber; even with the ramp squares underwater they can still dig them out. And not a drop of water will touch them... provided they clear out before you pull the lever.
Maybe a three-dimensional visualization is more appropriate for this.z 0 (last ocean level)
0BB▲ B is for
*▲☺иии -> to water
*▲▲иии -> reservoir
Red tiles are the final ramps that have to be dug out to breach the ocean. They used to be natural rock. The walls (0) were built in tiles that had been dug out to build the bridge. The dwarf will stand on an up ramp below the non-retracted bridge.
That's the whole catch: the dwarf can dig diagonally through the bridge. Once the ocean is breached, get the dwarfs out of the tunnel and open the bridge.
This also works for volcano breaching. This thread
has a simpler diagram with basically the same method.