Pidge being a researcher in marine biology and a secret cryptid hunter.
Keith being a lonely mer who enjoys people watching from afar.
Kosmo being a little reef shark keith adopted, that follows him around, they have a psychic bond so they can share feelings and intent, but not words.
Haggar is the sea witch, cause like, she is a witch is canon? maybe she could be less evil and more mischievious, she goes by honerva in this AU. She just wants to cause mischief not take over the kingdom and rule the mer... this is light hearted okay?
Shiro is part of the royal gaurd of mers, krolia is like queen triton or whatever.
Shiro is the over protective one like "keith you must be more careful, keith you'll rule this kingdom soon, keith stop running off!" and krolia is like "let the boy have his fun, he is still young"
Lance, hunk and pidge and the alteans are all human.
Shiro, keith, krolia and the bom are all mer.
Allura runs a marine research and rescue centre, where lance, hunk and pidge work.
Lance works with the sharks and rays, hunk works with the whales and porpoises, pidge works with the fish and corals
While taking samples of the local reef corals, some sort of accident (slips on a current, startled by something, idk) happens and her leg gets caught in the rocks and she cant get back to the surface.
Keith who was watching her from behind the reefs, rushes to help when he sees she is starting to slow in her struggles and is drowning, when he gets her to the surface he sings til she wakes up (cause i said so) but not like sing sing, more like whale/dolphin songs.
When hunk and lance hear the song and come to investigae keith is already gone and they find pidge coughing up water.
Keith runs into shiro, shiro does his hovering worry filled brother routine and keith is like "my dude, my bro, im fine"
Pidge avoids the beach for awhile and hangs out at the research center listening to whale song and porpoise song recordings trying to figure out what she heard when she was saved.
And keith, who hasnt seen pidge since the incident is worried and goes to haggar to get legs (cause duh we need legs to get the plot moving)
Shiro is all "you cant see the sea witch! keith bro no! she be sneaky!" and of course keith gives shiro the slip and goes anyway.
Honerva is in her spooky whale bone cave and agrees to give our boy some legs, saying "you have one week, if you dont get the girl to kiss ya, you will belong to me(or something idk)" so she takes his voice (cause this is a little mermaid AU!) and sends him to the surface with his legs.
kosmo is obviously sad cause he cant follow, poor kosmo...
And keith is now naked as the day he was born in the middle of a tide pool right when lance and hunk comes along to comb the beach for sea life thats in trouble (or whatever marine rescue does on the beach) Keith being a mer with absolutely no knowledge of human customs is extremely happy that someone came along cause boy does he not know how to use legs. (i can just hear lances squawking in my head from here)
After giving hunk and lance a heartattack each (they give him a jacket dont worry he isnt as naked now), they take him to the nearest building which is the research centre, but not before trying to get the mute guy to speak (lance holds a grudge cause he thinks hes being messed with)
Keith is happy to find pidge is there, hurray and pidge is very confused as to why a half naked guy who wont speak is trying to follow her around.
They keep him around cause they figure out he has a way with the marine life (he can telapathicly comunucate with them, so he can get them to work with him easier) They also believe he is either really dumb or lived under a rock. cause you cant have a little mermaid AU without mers being really bad at humaning!
conflict? well i guess them trying to figure out what is harming the marine life in the area, at some point it turns out to be sendak and his crew illegally fishing with illegal nets(you know the ones that are really inhumane) sendaks crew caught word of mers and are hunting!
They probably catch shiro which then turns into a rescue with the voltron crew raiding the boat and saving shiro, hurray and then yadda yadda kiss omg keith can talk, omg keith is a fish (lance squawking again) omg the whole coast is a mer community hiding in under sea caves, blah blah blah shiro has several anuerisms and krolia is just glad her son got a girlfriend
---will build on the conflict part in another chapter---