There are several recorded sightings of what are reported to be big ugly mermen in lake lonster lore. One such creature is said to inhabit Thunder Bay in Lake Superior (first reported 1792 and subsequently called a 'Merbeing')
I am attaching an illustration of a Vodyany, in this case a Russian 'Old Man in the Waters' in an illstration from my edition of Larousse Mythology, a drawing made by Ivan Bilbin in 1934, and in direct comparison to a bearded seal in Spitzbergen (Arctic Ocean). The bearded seals can turn up in the oddest places, most notably once recently inland in Florida, but also sometimes in the Baltic sea and even in Japan. In the article in question in Larousse Mythology, the location of Olonets in Karelia is specifically mentioned, although similar reports come from adjoining parts of Russia and nearby Finland.
This is not the only kind of Vodyanoy: in central Europe the name attaches to a sport of aquatic dwarf that wears raggedy clothing and a broad hat, and another version is described as being a large frog or salamander that sometimes looks like a log that moves against the current, and which seems to include the 'Pskov crocodiles' as one of the variants. This would be the giant salamander sort of creature, also known under a variety of other names in the Baltic countries.
But in the case of these Ugly Mermen of Karelia and Finland, I think we are plainly dealing with a type of large seal that has wandered inland. The same sort of creature could also be at the base of 'Merman' reports in the North Sea and especially around Norway. I remember reading in a book called Mysteries of the Sea that an identikit of merman reports was closely similar to a description of a walrus, minus the tusks.