Mermaid

Combat
Very fast movement in water, slow movement on land. High HP but poor magic. With the best health of any base species and good strength, mermaids are decent fighters even on land, but they're deadly against non-Aquatic enemies in the water. The mermaid's Aquatic trait effectively gives them Agile while fighting non-Aquatic enemies from the water, frequently allowing them to attack twice in a turn, particularly on the turn after they Dive to a water tile next to an enemy, or catch them in the zone of CreateWater. Non-Aquatic enemies lured into the water will also be afflicted with Soaked, weakening their attacks and reducing their armor. Mermaids naturally excel on water maps, but can struggle to maneuver into combat and position themselves on other map types. Mermaids can make their own water tiles if necessary at the cost of some of their scarce mana by using CreateWater, but using them in combination with a water-element MC, certain water spirits, or water-creating hybrids like the PlantMermaid can make positioning themselves properly in combat much easier. With just one water tile placed in the right spot next to an enemy, a mermaid can reach it by using Dive and enable their Aquatic trait. Mermaids have high HP pools and can even sometimes have a 0 AP self-heal ability in Regenerate, so they're great at absorbing damage, but they have no natural armor and still need support from a healer in long combats and expeditions. Some mermaids get WaterBolt, but unless they have good genes and traits for it, they might not be very powerful mages. A low power WaterBolt can still be devastating against lava-type enemies, though, and it can soak armored or heavy-hitting enemies to debuff them.

Overworld
Mermaids' Aquatic movement type allows them to quickly traverse water, but slows the party down on land unless a LandMount or AirMount of the proper size is in the party. Regenerate is usable for self-healing while on the overworld map, healing 25% of HP for 4 mana. CreateWater can turn a plains tile into water or a lava tile into stone at a range of 2, at the cost of 7 mana.

Breeding
Mermaids are the most fertile of all the base and (non-special) hybrid aquatic species, and female mermaids have arguably the highest reproductive potential of any base species unit once the fact that aquatic eggs hatch twins is taken into account. Mermaids generally don't have the magical ability to use their WaterBolt and Regenerate skills very effectively, but mermaid hybrids with better magic like the Siren or Water Angel can put them to good use if they inherit them.

Moneymaking
Mermaids' beauty is above average but not outstanding, and their only quirk with training sex skills is that they're unable to give footjobs (which doesn't really matter), so they can fill open slots in the brothel just fine but will be outshone there by other species. They have no particular advantage useful for collecting money or resources at home, but their aquatic nature makes them suited to exploring ocean portals for crystals.

Trivia

 * Their vaginas are a lot more lubricated than normal, so the effect works even underwater. This makes it possible to insert even the biggest cocks with relative ease into their tight pussy.