Merfolk and False Shipwrecks

Trade

Merfolk want metal, because you can't smelt anything underwater.  In most places, it is both illegal and taboo to trade metal to merfolk.  Those that do are sometimes called iron whores.

Merfolk mostly trade with vast quantities of delicious fish and/or weird artifacts from long-drowned civilizations.

Most nations of merfolk claim all the oceans, and forbid humans from sailing on them.  In warmer waters, merfolk usually dominate the seas (by agreement or warfare).  In some places (Abasinia, Basharna), this schism between land and sea is so strong that humans live by the oceans, but almost never eat fish.  Fish is called wet food, and is considered either a delicacy or a traitorous food (depending on whether or not we're at war with the local merfolk).

Cities

Contrary to popular belief, most merfolk cities are along the coastline.  (Real world fun fact: coral reefs are just as productive as rainforests, but the open ocean is about as productive as an arctic tundra.)  Some of the most productive pieces of coastline are where rivers empty into the ocean.

This means that humans and merfolk are in direct competition for the same pieces of prime real estate.  They both desire coastal cities.

And just as human coastal cities extend a short distance into the sea (in the form of docks, harbors, and shipyards), so do merfolk cities extend a short distance into the land.

Merfolk do this because there are certain things you can't do underwater.  Metalworking, for example.  Valdina, the largest merfolk city on the continent of Centerra, is often shaded by plumes of smoke from its smelters.

These industries are worked by either human slaves or human citizens.  (Both exist.  It's complicated.)  Usually the buildings will be separated by canals or moats (to make their city more defensible).  Sometimes the buildings will be on artificial islands.

Their cities usually have mazes of deep moats around them, studded with towers (emerging directly from ponds between the moats).  They do not build walls.

Warfare

Merfolk rule the seaways.  Every single human vs. merfolk naval war has ended with the human's defeat.

Their favorite tactic is just to swim up to a ship, grab hold of some of the barnacles on the bottom, and begin drilling holes in the bottom.  They require metal tools for this.

The most common human defense to this is simply speed.  A ship in full sail is much, much faster than a swimming merman.

The most common counter to this are blockades, nets, and traps.  A line of buoys stretched across the water will catch a ship that blunders into it, slowing it down enough for the mermen at the nearby fort to swim over and scuttle it.

When a ship runs into a ship-catching net, a sailor is quickly dispatched into the water to cut the ship free.

Sometimes the merfolk tie sharks to these nets to discourage people from getting into the water.

The zerino have been fighting the merfolk for so long that they have developed ingenious counters.  They cover their ships with spikes to keep merfolk from drilling holes.  Even when their ship is ensnared and bogged down, and they are surrounded by merfolk, they are safe within their dreadnoughts.  They dump poison into the water to kill the merfolk.

Stormsailing ships usually have a weather-mage on board.  Merfolk know to avoid these ships if there is so much as a single cloud in the sky, because these ships have metal masts that dip down into the keel.  If a weather-mage calls down a lightning bolt through the mainmast, everything in the water dies.

These are strong counters, but the merfolk counter-attack by throwing noxious sea creatures on board, releasing poisonous gases (produced in whale carcasses, which are floating alchemical laboratories), or--most rarely--with ocean mages of their own, who can summon whirlpools and giant waves.  (These get the most attention in sailor's stories.)  Or they simply fill your ship full of harpoons attached to parachute kites, which both slow you down and make your ship more visible to other merfolk in the area.

But the bottom line is this: if they succeed in slowing your ship down, they will eventually overwhelm you.  Once your ship is swamped (surrounded by many hundreds), they will swarm on board (clumsily, and with great difficulty) and hack you to pulp with shark-tooth axes.  They do this so they can capture your ship.

Ship combat versus merfolk needs to feel like Mad Max.  It's a chase, not a fight--because you'll lose if they catch you.  It needs to be full of weird weapons and weird counters to those same weapons.  Sailors need to be strapping on jellyfish gas-masks after the merfolk explode their whale corpse-bomb.

Merships

It is a fact that the fastest way to travel through the sea is to travel atop it.  If you want to be fast on the ocean, you need a ship.

The merfolk have long since learned to sail in ships, as the humans do.

Their ships are half-submerged most of the time.  When they want to be stealthy, they'll lower the mast and tow the ship.  When they want to be fast, they'll bail the ship out and sail it above the waves.

Sometimes they seal the ship, anchor it in a shallow place, and pull it down to the seabed so that it looks like a shipwreck.  When they want to give pursuit, they cut the lines and the ship erupts from the seabed.  Sometimes they are even angled so that the buoyancy forces the ship forward as it rises, so that it erupts from the water with forward momentum, at full speed.  (Like holding a bodyboard underwater, angling the nose slightly up, then releasing it.)

Merships tend to be inferior to their human counterparts (inferior wood, collapsible masts are much weaker), but in merfolk-controlled areas, there will be large areas full of ship-catching nets, sandbars engineered to be invisible, and pyramids of rocks stacked just beneath the waves--the perfect height to punch a hole in the bottom of a fleeing ship.

These places are called mazes.