There are plenty of myths surrounding the Rivermaids, enough to fill not just one volume.
Odd phrasing...
Suggestions to improve:
"There is a myriad of myths concerning the Rivermaids, far more than one volume can hope to contain."
"There are numerous myths surrounding the Rivermaids, enough to fill at least half a dozen tomes with ink."
"Most are describing them as young beautiful women who ensnare young men just by being themselves, sitting around on rocks and grooming each other."
"Just by being themselves" is a strange way to convey that sentiment...
Rephrasing and added details may improve sentence...
Suggestion:
"Most describe them as young women whose overwhelming pulchritude enchants the hearts and minds of any man who happens to encounter them lounging among stones and boulders smoothed by the softening touch of running water."
"Often a relation with a young man is narrated which always ends with the death of the young man."
This is confusing...
Try reworking the sentence structure...
Suggestion:
"Typically these tales are anecdotal in natural, extracted from a relative of some young lad who suffered an unfortunate demise. The conclusions of stories spun by such people often mirror the somber tragedy of their past."
"There are exceptions though: one where the young man survives but lives with a Rivermaid and other, where the drowned ones live on as a Rivermaid themselves."
Fleshing out and restructuring if author feels it is necessary..
Suggestion:
"There are exceptions of course, in one tale the young protagonist manages to survive his encounter with a Rivermaid by virtue of pure tenacity winning himself a peaceful life with her. Another tale claims that those who drown under a Rivermaid's charm shall become Rivermaids themselves."
Both myths originate in Salsair: The story about Tylunn, a young man who fell in love with a Rivermaid, eventually leading him to abandon the responsibilities of his prior life so that he could become her lover.
The other is about the gods being benevolent and allowing drowned citizens of Salsair to have another life as a Rivermaid. The full stories can be found in the Salsair entry.
The previous two sentences are reasonably well composed...
...
Mhm...
I'm too lazy to do the other paragraph...
...
I hope this helps...