I have fortress carved into walls of deep cave with big lake (only connections to cave is some carved fortifications and one open area for fishing). Fortress is ~15 levels above my main smelter area. And I wanted to do a small obsidian casting area. For this I had to tap into cavern lake from one of the fortress tunnels.
My setup was simple: water shaft with lever-controlled floodgate, and grates covering a hole to casting area. The miner has to break the outer wall of lake (3 z-levels deep in the water, by the way) and retreat (or be pushed by water) few tiles back, to the grates area. I ordered to dig through a wall and paused to step through it if something went wrong. One of my two miners are breached the wall and the fun ensued.
Well, not the kind of fun that bring the entire fortress to its knees. The water filled the shaft and casting area almost instantly. Moreover, the miner wasn't pushed backwards: he walked against the flow and out to the cave full of blind ogres and rutherers (which were awfully close to new water intake point). When miner decided he'll take a swim in the lake I ordered closing the floodgate and it was closed just in time to save half of fortress from drowning (including smelters area below), but the miner was now sealed outside. He swimmed to the muddy shore ~30 tiles from the fortress, without armor, with a pick as his only weapon. I immdiately turned off most of his labors and ordered him to dig a shelter, and wall himself inside so he has time to dig into one of my tunnels without rutherers interrupting him. Sadly, he decided to construct the wall while standing outside of his new bunker. Then I proceeded to dig channel to separate the shore from the rest of the cave. Right now he dug out the channel and in process of removing ramps in it. I designated shore as water source so he can drink (but will he drink?). At the same time, fortress dwarves are building a temporary rescue bridge to the same shore. Rutherers are walking around, but hasn't got to miner yet (the cavern is full of twisty little passages near this shore, and I have to channel only these to keep the cavern fauna away).
Now the lonely survivor is finishing his improvised moat, rescue team is buildig a bridge, militia is ready to fight (though I haven't met rutherers before and not sure if I want to battle them with only bronze weapons and armor).