What adaptations are needed for human-derived sea-dwellers?
My people are not the classic half-fish, nor are they whale-tail-people - they wear ankle-length swimming skirts that bind their legs into a hydrodynamic shape, and flippers. On land they wear loose skirts and walk almost normally. (The skirts cause some misunderstanding and false rumors, but under their clothes the merfolk aren't that different from us. Bonus points for the possibility of hybrids not well-adapted to either life.) Decorative clothing is flat and clingy.
Genetic engineering technology was a few centuries advanced from ours, but is lost now -- the species was designed, created, and possibly tweaked a bit for a few generations, but now breeds true naturally. (Bonus points for occasional throwbacks who are not well-adapted to either water or land.) Root stock was carefully chosen for genetic diversity but maybe the mix needed tweaking with experience.
Magic is available, but is not something used in daily life. (Think of magic talent like musical talent. A lot of people enjoy singing. Some sing well enough that other people enjoy it. A few can make a bit of money playing gigs. Very few can make a living. A handful are important cultural assets. So important public works can be magicked a bit, but not your average home, reef ranch, or kelp farm.)
There's already a great question about their building methods, BTW. There can be a few storage buildings underwater, but people need to sleep where they can breathe air even if the entrances are underwater like beaver homes. Most human coastal cities have a sea-folk quarter, especially in tropical areas near reefs.
This needs to go further than the (heavily disputed) lake-dwelling shellfish-gathering waders proposed as human ancestors by Sir Alister Hardy in 1960, popularized in 1972's feminist The Descent of Woman and given a more scholarly treatment in 1982's The Aquatic Ape Hypothesis
So far I've got:
- Lungs -- extra capacity and pressure. Bigger ribcage.
- Gills -- aren't feasible, see this question
- Hair -- something between human eyebrows and seal fur
- Ears -- streamlined
- Genitalia - retractable/covered
- Body temperature -- cooler in water, warmer in air, but always lower than base stock humans (Camels use a similar scheme, probably to reduce water loss.)