I do not know, but if I had to take an educated guess, I would suggest they may be trying lure people to their deaths.

[ Spock thinks of the old lore of sirens and other such sea creatures. There is nothing logical about this situation and Spock struggles to find his footing within that. He's not exactly shaken by the undead, but he is tense and alert after having been nearly dragged over a rail himself earlier. ]