On the whole, I was drawn into RM's stories more quickly than PD's (my favourites are "The Sea King's Son" and "The Water Horse"), although after repeated exposure I've developed some liking for two of his three. McKinley's stories herein seem to me to have more detailed and polished world-building. None of the six, to my knowledge, have been published previously.
"Mermaid Song" (PD) Setting = very like Puritan New England. (I'd have enjoyed it more if PD had simply made it an alternate Puritan history.) While the mundane setting may be off-putting at first, the sea-people's introduction is well handled when it comes. In a way, this is two stories - a family tradition (handed down from mother to daughter) and the story of the protagonist, young Pitiable Nasmith, left with her maternal grandparents upon her mother's death in childbirth.
Near the end of her life, Pitiable's grandmother tells her the story behind the most unusual of her songs - how their ancestress Charity Goodrich really survived shipwreck upon arriving in the new world as a girl. Although the People's culture isn't fleshed out much, the first contact scene between Charity and her sea-children rescuers is realistically detailed. In a neat reversal of some sea-people stories, the air-breathing person was a pet, kept in an undersea cave with no way out.
The present-day story turns grim when the grandfather takes to drink after his wife's death, which seems to have quenched what little of his heart survived his daughter's passing. Eventually he takes to walking along the seashore, and finds something that only Pitiable has learned to recognize, shaping up to a possible reversal of the secret tradition.
"The Sea-King's Son" (RM) Jenny, only child of a well-off farming family, grew into shyness as she grew up, and never let on that she had fallen in love with Robert, a good-looking younger son of another farming family from a village on the far side of the harbour separating the small towns they live in - a harbour under a curse by the king of the sea people, to avenge an injustice inflicted by the land people in the days when the two races had dealings with one another (though only a trade in luxury items, never friendship, each race considering the other too alien to grow close to). But when Jenny's parents make plans to send her away to the city for a season, in the hope that she might shake off her shyness, and perhaps find a good husband, Robert finally makes a move - for love of Jenny's inheritance rather than for her. But late in their courtship, Jenny makes an unannounced visit alone to Robert's family home, and what she learns there is more terrible for her than any ancient tale of sea-curses, and drives her onto the shortest road home - the direct route across the harbour.
"Sea Serpent" (PD) I was disappointed with the initial scene-setting, although the wave-riders eventually won me over a bit. The conflict between the New religion's chief god and the Old's chief goddess comes to a head as the builder of a new temple seeks building stone taken from the goddess' shrine (which seemed unoriginal). The magic-working temple-builder forces the neutral wave-riders, worshippers of the Sea God, to help transport the stones. The details of the minutiae, practical politics, and ethics of the wave-riders' work make the latter portion of the story a decent read.
"Water Horse" (RM) "This island is a strange place...a threshold between land and water; and the boundary between us is striven for, and fought over, and it shifts sometimes this way, and sometimes that...it is over this one island that the war is fought, and if once we yielded, then all those lands behind us - farther from the boundary we protect - would immediately come under threat, and they have no Guardians. We are the Guardians; and here we hold the line." So says Western Mouth to her inland-born apprentice, Tamia, who began her training at fourteen as do all apprentices, and can't help worrying that she's not really suitable for the work. But Western Mouth was a very old woman by the time Tamia came along...When Western Mouth has a stroke five years into Tamia's apprenticeship, the defenses are torn open, allowing a creature of sea-magic to slip through that Tamia must face in her Guardian's stead.
"Kraken" (PD) Somewhat similar to "Mermaid Song", although the two humans swept into the water are saved by more supernatural means and for more complex reasons. The protagonist, a young sea-princess indulging in her last rule-breaking before coming of age, runs serious risks to try to return them to the upper air.
"A Pool in the Desert" (RM) The only Damar story herein - not surprising, for a country bordered by desert in the more recent ages of the world. The protagonist, a present-day Homelander (not unlike our own present), begins dreaming of a time so far in Damar's past that it has become legend, and finds it far more like home than her parents' household, with their stranglehold on their children.