I read this in a science fiction anthology which I checked out from a public library in Indiana sometime in the early 1980s. I'm thinking no later than 1985. I don't know how old the book was when I saw it, nor when the story in question was first published. I may have run across a copy again in the 1990s, and thus refreshed my memory a bit, but I'm not sure of that. I remember the plot pretty well, but nothing about the author's name, nor that of the main character.
The main character is a scuba diver or something similar. (The word "scuba" may not have been applied to whatever gear he was wearing.) Many years ago, his beloved brother drowned -- I forget whether the brother had been diving at the time, or if "his ship was sunk in a hurricane," or what -- but that doesn't keep the protagonist from enjoying himself down below the surface of the water from time to time.
Early on in the story, while exploring underwater, the protagonist spots a human skeleton which appears to have been resting in one place for a long time. The protagonist immediately remembers some words sung by the character Ariel in Shakespeare's The Tempest. (A play which I had never read or watched, at that time, so this story was my first encounter with these memorable words.)
Full fathom five thy father lies;
Of his bones are coral made;
Those are pearls that were his eyes:
Nothing of him that doth fade,
But doth suffer a sea-change
Into something rich and strange.
Sea-nymphs hourly ring his knell:
Hark! Now I hear them – Ding-dong, bell.
The protagonist then mentally compares the descriptive details from the poetry to the underwater tableau in front of him, thinking such things as "Full fathom five? Well, not quite that deep," and so forth. I think it may have been explicitly stated that this couldn't possibly be the skeleton of his drowned brother.
Thus far, the story could be happening in "the real world." But now it gets weird. The protagonist comes face-to-face (using the word "face" somewhat loosely) with a humanoid-shaped creature or conglomeration which, I think, appears to have been made from various smaller units of marine organisms -- perhaps a fish here, some seaweed there, and so forth, as if someone were assembling a man-sized jigsaw puzzle out of biological odds and ends, but not making much effort to create a convincing imitation of a human body. This creature (assuming, arbitrarily, that we should regard it as a single entity) stays underwater throughout the story, but somehow manages to persuade the protagonist to follow it further out from shore with the intent of being helpful.
I think something is done which (temporarily) allows the protagonist to breathe comfortably while underwater, so that he won't be facing a hard-and-fast time limit, mandated by the capacity of an air tank, during his attempts to be helpful to whatever society the "guide" represents. But I may be confusing that part with a plot device from another tale of underwater adventure which I read as a boy.
The protagonist fights some sort of sea-monster which may have resembled a giant clam, and kills it. I can't for the life of me remember what was implied about why the sea-dwellers who sent that "messenger" couldn't have ganged up on the monster and done the dirty work themselves. Perhaps the clam was powerfully telepathic, but less effective against the brains of land-based organisms? (That's more of a wild guess than anything else.)
Having done this, the protagonist is then guided to where a giant sea-shell opens up to reveal what appears to be, more or less, a beautiful young woman -- water-breathing, but looking much closer to "human" than does the original guide-creature -- and the protagonist gets the distinct impression, perhaps via limited psychic communication, that he is supposed to mate with her, right here and now! He wonders if this is supposed to be a special reward for his valiant service, or one last favor they are requesting of him (if the underwater civilization has plans for his genes), or something else. But whatever the motives behind this offer may be, he doesn't feel comfortable with the idea of a one-night stand with a mysterious, perhaps genetically-engineered, marine creature that only resembles a member of his own species . . . and so he turns down this rare opportunity.
That decision does not go over well. In fact, it stirs up a great deal of animosity. The protagonist starts trying to fight his way back up to the surface, but now he is having serious trouble breathing. As I said, I'm not sure if he was being "magically" aided in breathing underwater before, but if he was, the magic is now being revoked. Or, if there wasn't any magic, then maybe someone or something rips away some of his scuba gear. Either way, he figures he'll be dead before he makes it back up to the surface and over to dry land. Then he feels a strong arm wrap itself around his torso from behind him, and someone essentially carries him up to the surface and shoves him up onto a beach, but disappears below the waves before the protagonist can turn around and get a good look at the rescuer.
Someone else -- an ordinary land-dwelling human being, this time -- comes along and speaks to the protagonist, asking about what has just happened. Our hero says something along these lines: "I wouldn't have made it back if I hadn't been helped. I think . . . it was my brother!"
I don't remember whether the author's name was familiar to me at the time I first read this. It may have been one of those cases where an author only dabbled in SF/Fantasy a few times in his life. I'm sure the story is not collected in any of the many volumes I own that collect much of the shorter fiction of such big-name authors as Poul Anderson, Roger Zelazny, and Gordon R. Dickson.
I did make some effort to Google for this story before taking the trouble to type out that plot summary. As a result, I'm certain it is not the story "Full Fathom Five My Father Lies" by Rand B. Lee (the answer to this old question). Nor does it seem to be any of the fictional works which Wikipedia, on the following pages, lists as having recycled some or all of those famous lines by Shakespeare: Ariel's Song and Full fathom five (catchphrase).
With that said, does anyone recognize this from my description?