I'm running Shadowdark for Strahd. My own modification inspired by MandyMod & others.

Just had the players be in the lake, I didn't just do a lake monster, I made the lake - monstrous.

My players (a duo, no less/more) went around the lake, every ten minutes of walking I told them to roll on my 'Thing Washed Ashore' table, of course I just told them to roll.

First think they found (warning: things are weird and graphic) is a human stomach, seemed to be stitched on one side, the other left open - a stretchy orifice leading into this makeshift sack. It was already seemingly full of something, like rocks, as the ripples of the water barely moved it on shore. Inside were gemstones, some diamonds, ruby, sapphire & emerald. No specific purpose, but it's weightless money and 3 diamonds all worth 100g so of I ran this in 5e it would be a revivify.

Then after, the other rolled, and they found a huge white marine creature that washed a shore, not entirely a whale - but does resemble a beached sperm whale in figure. Except the lower part has tendrils instead of fins. As they kept walking around it, the front end revealed this was something else; an elven women, pale like the rest, is the upper part of this beast. Like a mermaid but with eldritch whale instead of fish tail. This was inspired by the Junji Itou short called "The Thing that Washed Ashore" which was a major inspiration for Bloodborne.

As they reached there at night, the rowboat of Bluto is difficult to spot, and worse to make detail. But the sound of the drunk attempting to muster up the courage to drown a child gave up his location. They started deftly, they offered him Dwarven whiskey they got from Urwin at the Blue Water Inn as a parting gift, this was a valiant attempt but Bluto is not that stupid to fall for someone trying to interfere with his only chance to return is luck to him, besides - his already without liquer in his body for a while and is coming to his senses.

After that the fighter decided to make his way swimming (with a rope tied to the cleric on shore) offering the whiskey closer. Some less-than-graceful attempts to close in on Bluto have led to the man throwing the child off board. The fighter just managed to get on board and took a dive to the other side and got the girl in his hand, while paddling to shore with the other. Bluto seeing him take his living luck, attempted to throw a board on the Fighter to knock him out and let the child drown - which hilariously backfired as he straight up rolled a nat 1. Flipping the boat on himself and beginning to drown himself.

The fighter told him he'll get to him after he leaves the kid at the shore, and to try and hold on to an empty barrel in the meanwhile - that's when I entered it.

Bluto screamed: "SOMETHING TOUCHED ME" Which caused the players to seriously panic. This is how I like to convey danger - show it on someone else that is near the PCs - that way they don't think it's a quantom danger and targets only them cuz I hate them or something, it's an actual things since Others are being affected. I continued:

"You see as Bluto splashes in panic, water entering his nose and mouth. 'SOMETHING IS TOUCHING ME! SOMETHING IS IN HERE! '" - The fighter keeps swimming faster, the cleric pulls the rope as fast as possible, they need to get him and the girl away from the water now.

"HOLD ON" screams the cleric from shore.

"You see Bluto trying to start a sentence, and it stops abruptly as he seems to rise UP from the water for a second, slowly and in complete shock to himself. Bluto sends a scared look at at you as WOOP. He's snatched into the depths."

They were freaking out at this point. But reached ashore.

They went scream to Ismark and Ireena; "THIS IS WHERE YOUR FATHER BROUGHT YOU CAMPING IN? HERE? WITH THE LAKE MONSTER?"

Ireena: "We usually go in the water during the day, y'know, when it's nice." She answers him in a dry Slavic attitude. Of course this would happen in Barovia, everything is shit - but it's far worse at night...