The area's merfolk are aware of Cetus' existence and instruct their younglings to stay away from this particular area of the bay.
This trio (the third was dragged there by peer pressure tbh) wanted to sneak a peek at the enclosure of this "giant mer with a condition that the humans are in charge of and we've been told to stay away from". Arion there got a lot more than he bargained for because he wanted to look cool (and also did not know Cetus was that big or close by. xD)
It might not be very clear from the drawing but that's a "fence" of batteries anchored to chains. Similar to how shark deterrents work. Very effective in keeping Cetus in.
Also keep in mind, the mers are speaking Mer so Cetus has no idea what's being said. Language barrier on top whoop whoop!
floating in donut at the beach while they play jaws the revenge on an old projector and i can't stop thinking about that one scene in Creature from the Black Lagoon where the creature is actively stalking the mc by swimming below her and like. i need that pred/prey energy with Price. being hunted in the water, dragged to his cave. his little human conquest.