Only with my latest Adventurer have I been given (and pursued) a sewer-bound quest target. Two in fact, in a walled town whose keep has an extensive dungeon with no (as far as I can see) connection to the sewers. I've accumulated so much wealth from raiding the dungeons that I'm packing it in multiple money-pouches, and one stack of (high value) coins is so large that it'll only fit in a silk stack. There was a kobold guard in there, who I left in peace, with loads of dead bodies and I only bothered retrieving weapons and gems for sale in the town above.
The sewers are 7/7 flooded everywhere[2] (because I've never explored them before I took this for typical behaviour), appear to be single level (at least where the terrain doesn't make higher chimneys to grates) and I'm not ready to turn myself vampiric to (I hear) remove the necessity for breathing.
One of the targets I managed to find just within the limit whereupon if I dropped absolutely everything I was wearing, and all but my main weapon of what I was carrying, I could swim inside within sight of the Perp (unsneakily) and then rush back out with him/her/it following me, then once I was no longer Drowing I could safely clonk it on the head as it emerged. (My swordswomanship[1] is near legendary, my swimming almost that, and I'm not sure what "Legendary+value" it would be, but my sneak was, last time I looked, being advanced upwards against a "/4200" target for the next level, but obviously I wasn't using my sneaking on this occasion. Although I can brush right past flocks of ravens in the open plains without a rustle of a tail-feather...)
The other target is further away. I can see them through a grating, while exploring the town (as with a number of cavefishpersons of a non-vampiric nature who keep wishing to threaten me but can't do anything about that) but nothing thrown will reach them, and of course (I say, but maybe there's something I've missed) I can't utilise the grate-hole in any way to either jump into the sewers at that point or make a one-way dash past this Cave fish man/woman vampire (I forget which gender) to get a breath and then fight within easy reach of air and then make a return dash to the outside again.
Anyway, I've long left the second target. Wandered the world some more (a tiny fraction of it, but many sites of many kinds... I think I requested a lot of sites on worldgen). Generally only killing quested creatures, but occasionally (because it's easier) dispatching a cave-dweller with no notable claim to have been killing anybody or their animals so that I can happily sleep in their caves. Where possible I've been butchering, but then that gives me the quandry regarding not wasting meat (or offal). I tend to put their less useful end-product in a pyre (or light a campfire on the spot that I've let it lie), to mirror my long-time practice of putting vampire and werebeast(/human-with-werebeast potential) corpses in similar pyres, just in case something in the game tells them to arise.
I've generally avoided bogeymen (either by sleeping in shelter, gained by ascent or conquest, or just by sneaking past them), although I've got four of them in my kill-list as well (a near-dawn encounter and some fortuitous geography meant that even when my cover was blown I could pick my fights for long enough to ensure that the four I killed weren't followed up by a luckier bugger of a nightcreature who could have done more than merely bruised me).
And, at some point, possibly after some more swimming practice (if that actually helps any) I might wander back by that walled town and see if I can dispatch that remaining fishy vampire, but I'm rather planning on sweeping over (or under!) a range of mountains, first. And two sites ahead, in my current plan of quest-fulfilment, there's a cave (or six, in close proximity) with half a dozen vampires needs dispatching, quite close to a cave (or maybe more) with a slightly smaller number of werebeasts. Which I feel confident of cleansing, but you never know...
[1] Female human, pretty much deadly with a sabre, has been training up the lashing skill with a silver whip, though...
[2] Almost. There are four separate river connections into the (presumably connected, certainly as far as my surveying from the town above is concerned) sewer maze, and the most downstream one has some 5, 6th/7ths variation in level, but not low enough, consistently enough to get much further in (against a current, it seems). And, besides, it's the furthest end of the system from where the more elusive fiend has ensconced themselves. There's one inland entrance. I slightly error of geography/geology means that one of the sewer-tunnels that peaks out under the walls in the opposite direction from the river (but at the same end as the last target) has a diagonal-break in its upward tunnel to the grate, such that I can Alt-move diagonally across (and down, if I don't want to make a splash) into it, and get back out again the same way. But it's even further to the target from there than from the nearest river-exit, with 7/7ths all the way (that I can see) along that route.