The setting; there’s a large population of merfolk living in the ocean, and a developing seafaring human nation on the neighbouring coastline.
These merfolk are entirely aquatic. They have gills and not lungs, they would suffocate if removed from water. They can survive on the surface only as long as they can hold their breath, and their bodies are totally unsuited for any movement on land. They have upper torsos, arms and heads that are broadly human-like, and fish-like tails. They live in fairly deep waters and in scattered colonies under the ocean. They farm and hunt sea creatures, they harvest the reefs, and their villages are at least a hundred foot below sea level.
These merfolk are well-used to fighting off large predators like sharks, or fighting skirmishes against rival underwater colonies.
Technology-wise, humans would class the merfolk as being primitive. They have no advanced tool-making or metalworking (because, well, no fire), but they craft coral weapons, and they can bind seaweed into rope. They primarily wield harpoons and spears, but some have crude crossbows. The merfolk are very good at farming fish, their society is very organised.
In the past, humans and merfolk have got along pretty well. They’ve interacted occasionally, but merfolk have had no interest what happens on the land, and humans don’t care for under the sea.
The problem comes when the human society begins to develop and expand quickly. Their towns become cities, they start building larger ships, and the human nation starts to trade and interact with other nations around it. Imagine these people as culturally similar to early Vikings.
To power this expansion, the humans have begun very heavily fishing in what was previously merfolk territory. Larger populations need more food, and the ocean is the only place they can get it. In particular, human whalers start to target schools of whale and dolphin pods, which seriously upsets the previously docile merfolk.
The humans aren't overfishing the environment, but the local increase in fishing is the point of contention.
In retaliation, the merfolk start sinking ships. Raiding groups of merfolk warriors use long ropes of seaweed to bind the ships’ rudders and anchor them in place. From then, the merfolk use coral saws and blades to hack through the hulls from below, and they drag the helpless ship down. Any human that jumps overboard is promptly slaughtered in the water.
This is difficult work for the merfolk (sawing through hard wood with crude materials isn’t easy), but the human sailors have no real way of defending against them. Harpoons work somewhat, but the merfolk have the huge advantage of movement under the water compared to the men trapped on boats above. The merfolk successfully sink several of the human fishing and trading vessels.
Back on shore, this is an outrage. The human king stamps his boot, and says this is enough. The humans declare total war on the merfolk.
The only problem is… how?
How would the humans fight against the merfolk? Submarines don’t exist, and diving with weapons would be foolish - so how can they launch counter-assaults against the merfolk’s underwater villages? Drop barrels on them? Poison the water? They have no idea, they’re looking for a way.
Is there any means to protect their ships from an organised attack from below? To trade, they must travel through merfolk-infested waters, but all of their ships are (literally) sitting ducks for the merfolk.
What do they do?
For answers, assume that this is a low-fantasy setting. Solutions that rely on magic are possible, but discouraged.