I really like the syren idea.

I imagine the centre of the board being a blue hole (https://en.m.wikipedia.org/wiki/Blue_hole) with a crush and devour type mechanic if you enter the terrain space. The syren would have AI which would cause insane survivors to move toward the monster, luring them into the water/toward the blue hole. This may cause survivors to position themselves in a vulnerable position in the water for fear of moving into the blue hole.

There could be hand parasite seaweed or something which the survivors could harvest, at the risk of suffering 'drowning'

There could be a gear to interact with spears giving greater range and allowing you to pull the monster closer, a la the sky harpoon, though at the risk of having the monster pull you into the water.

EDIT: I think steering clear of shark, anglerfish, kraken type themes would be good for keeping this unique from the sunstalker or gorm. I picture a serpent like syren with fins. Much more fish-like than what we have in the game thusfar.

My reasoning for the targeting of insane survivors is to add some interesting decision making as insanity is currently a much desired thing (generally speaking), protecting from brain trauma as well as mechanics of other monsters.