Weakspots under the sea
A famous Dutch story tells how one sailor heroically jumped over board, swam to the enemy ship and drilled some holes while clinging to it to make it sink. If memory serves correctly, the captain had some beef with the sailor and didn't save the sailir from the water afterwards.
The story shows something important. The vulnerability of the ships hull. Although the story always struck me as odd and nigh impossible, the mermaids should habe no problem.
The tools an 8m fish with arms and human intelligence can wield will be big and strong enough to penetrate hulls. They can also stay out of harms way under the hull, making them hard to stop.
Not only the hull is vulnerable. The rudder is also highly susceptible to damage. They can make the ship unsteerable or even steer the ship themselves, navigating to open sea and leaving them there to rot. Or they steer onto rocks or the like.
The mermaids are pretty powerful. Swimming up to a ship, drilling holes and making steering impossible. Next is attrition. Even if the ship can repair, they can't do so indefinitely. Not too much attrition is needed to make the ship stop to float amd become an involuntary submarine.