Make no mistake, this isn't as simple as it looks... and yet, it is. Very simple. That's probably my greatest gripe with "Still Waters", it's a bit too simple, and a bit too complex, in all the wrong places at the same time. Namely, it feels a bit like the Cthulhu elements are tacked on, and the premise is a bit linear - go check out Davies household, find the clues, go deal with the Rosethornes. The parallels between the Davies and the Rosethornes make or break this simple plot, allowing the discovery of how human darkness can falter into Mythos darkness quite easily. As for complexity, Sweet Galloping Carcosan Christ does this thing have a shitload of clues - if you are making props for this scenario, you WILL have your work cut out for you. The Rosethorne file alone took me a good hour to make up. I'm fairly sure Masks of Nyarlathotep has fewer props-to-plot than this scenario has, and it's a campaign! Needless to say, props make at least part of this scenario, and you should put your all into making them.
So if props make this story partly, and flavoring the town makes it partly, what completes this scenario? Hands fucking down, the Rosethornes themselves. They are very, very dangerous; they might as well be the first real threat players will face in a large-scale campaign. These sisters are tough, they will fuck up, kill, and mangle players if they are not obscenely lucky, expect at least a few deaths to happen if the dice gods aren't playing nice. I am absolutely serious here in how deadly these "goofy" maniac sisters are; one player's character was nearly left behind as the skiff when going down the grate into the river, and only survived and escaped with his life by the good grace of the die for his DEX roll somehow going from 80 to 00 at the very last second, the very difference between a failing roll of 85 and a hard success of 005. I was so damn impressed that I didn't even make him roll Jump to get on the moving skiff!
Playing the sisters was an absolute riot for me as Keeper, and really gave the whole thing a humor-horror interlude. My players didn't know whether to be terrified of the exuberant and manic octopoid sisters who wanted them as permanent (and very dead) house guests, confused by them not outright ripping their faces off while complementing them, or both! One person ended up smooched by a sister when he failed his Charm roll critically. One character had to fake enjoying his "dinner" of poor Phillippa Davies' leg. Yet another, our black musician, was traumatized to have a sister crush his beloved saxophone in one fell swoop. I can't stress this enough, if you do this scenario, for God's sake make sure the Investigators tangle with the Rosethornes! You will laugh your ass off seeing the absolutely confused, freaked out looks on your players' faces as they deal with the two nicest Mythos monsters that will ever try to murder them dead. The only issue with this is that a horror-humor blend like this one really may not gel well with some campaigns. It will really all depend on the Keeper, the story, and the players. It's an odd fit that needs to work just right to keep its zany feel in step with other campaigns, but in some it can be a point of interest or much needed shake-up. Sometimes, such an interlude is just what the doctor ordered, or the boost a sadistic Keeper needs to really twist the knife in the players' backs.
All in all, despite a respect for and fondness for this campaign, I am... shockingly ambivalent to "Still Waters". It's nothing personal, scenario, you're a nice guy, but I'm just not all that into you. Sure, it's nice to see aspects of Cthulhu in play in the game sharing his namesake, and the horror-humor blend with the Rosethorne encounter is an absolute riot to play through, but it's just not a great fit for most horror campaigns, which is basically every CoC campaign, without a good amount of Keeper effort and the right type of cooperation from players. It just might not be serious enough, or compelling enough, for some groups (especially after something like "The Spawn") unless you make it a nice interlude between other adventures. The time it takes to make props for this one, and there are entirely too Goddamned many of them for a one-shot, may be a deal-breaker for the size of this scenario, and overall this works better as a one-shot. The Rosethornes are hilarious, but not quite enough to save this one from the "In Case Of Missing Players" pile. Preferably, keep it a one-shot and use it as an aside for when some players just can't make it but others can. If you do run this one, run it with a large grain of salt, preferably one tinged sea green and carved into the loathsome shape of a certain winged, dragon-like, octopoid monstrosity. 5/10 slimy book-napping tentacles.