Someone more deeply learned in ancient folklore probably has more to say to about the deep connections of similar ideas, but I wanted to correct one thing from the question.

Näcken is an river/inland water spirit, usually used symbolically for the dangers of rivers, e.g. drowning. Especially children are said to be at risk from being taken by Näcken. "Stay away from the river or Näcken will take you" such stories.

Sirens rather represent the dangers of the ocean. On the sea you would not be concerned about Näcken.

While the two are broadly compatible as manifestations of dangers with water Näcken is more analogous to Greek najads, not Sirens.