Quite how often these pesky assh*les will bother you on your adventures around the realm depends on how close you get to the shoreline, with Drowners being a common enemy that will pile on Geralt throughout his time wandering in the game. They're blue, slimy, pot bellied, and have some horrible tendril-ridden crotch that defines them as both creepy and having spent far too long in the water - most of all, however, they're just annoying.
Stumbling across a few drowners is part and parcel of The Witcher 3, and their backstory is far more tragic than the shambling monsters would have you believe. In Slavic legend, Drowner was a man that committed suicide by drowning himself in a river, only to become a restless spirit in the afterlife that desired only to pull someone else in to take his place in the waters. Since suicide is a sin in most religions, this is a penance for having done wrong spiritually - like a fee to pay to the devil.
More widely, they can also be linked to Slavic Vodyanoy, a supernatural, amphibian-looking creature that drowns bold swimmers should they come near his lair, causing distress to local communities near rivers.
The aggression of Drowners in The Witcher certainly backs up this desperate ploy to pull Geralt to his death, with their appearance akin to the drowners own description of a man with green skin, long hands, and bulging eyes. Still not sure about that tendril d*ck though.