My first underground river led to some pretty horrible lessons. It was full of lizardmen, snakemen, and olmmen, and after one of them climbed out and massacred a peasant, I knew that it was time to mount a campaign against them.
I bricked up the opening to the river, then made a four-step assault plan: (1) Carve out a sizeable room near the water and station two squads in it (but don't expose it to the river yet); (2) prepare a ramp so that overzealous dwarves who dive in the water can climb out; (3) dig out one subterranean tile to expose the ramp to the river; and (4) dig out the aboveground tiles that still separate the squadrons from the river, so that every soldier can get in on the action. None of them can swim, but if enough of them participate, they can finish the fight in time to escape without drowning.
The problem is, my miner (a card-carrying Teamster) clocked out after Step 3 and went back to his elegantly-appointed quarters for a couple of weeks of deep slumber. Then a squad leader, appropriately named Rimtar Cryptfasten, happened to glimpse a lizardman through the narrow opening, and dove straight underwater. His cries of "Follow me to glory!" must have come out as "glub glub glub," because the other soldiers just sort of shrugged and stood around waiting patiently.
Rimtar actually fought very well, for a while, surrounded as he was by slimy critters. But eventually one of them managed to brown his thumb, and he was paralyzed. From that point I could only watch in horror as my drowning champion was slowly torn asunder, one tiny dwarf chunk at a time. It becomes amazing how many chunks a dwarf can lose while still having all his limbs; I lost count. I tried in vain to persuade my other dwarves to come to his rescue, but none of them could be made to notice the boiling bloody froth lapping at the top of the ramp.
I knew the cause was lost when an olmman chewed out Rimtar's eyes, and I called off the rescue effort. Interestingly, even though he was underwater for a very long time and eventually his throat was torn out, he finally died by bleeding to death.