Hi!
I’ve been searching for this book, on and off, for years…um, decades now. (Wow, that’s just…sad.) Anyway my latest google search lead me to you. I’ve tried to organize this semi-coherently, but I read it as a kid and my facts may be fuzzy.
Title/author/cover: no idea.
Publication: before 1992, likely before ’90. I’m guessing sometime in the ’80s.
Age: MG (shelved under Juvenile in the public library, where I found it)
Length: between 150-300 pages.
Style: anthropomorphic animals in their communities. No humans. No illustrations that I remember.

 Protagonist: A young (teenage?) male otter with a snakeskin eyepatch, who is high up in the otter tribe—the chief’s son, I think. He’s a quiet loner who doesn’t fit in and no one knows what to do with him.
Subplot #1:The community wants the loner to marry this girl otter, but she doesn’t want anything to do with him. She thinks he’s a poser, and wears the eyepatch to be “cool”. Besides, she’s in love with this other guy. She discovers late in the book that no, actually, he wears the eyepatch because he’s missing his eye. They become friends, though at the end the girl marries the other guy. We see the wedding, it’s one of the last scenes in the book.
Subplot #2: The loner falls in love with a fish! The fish cares about him, but doesn’t love him back. Not the way he loves her. She’s mostly like, “dude, seriously? This is insane. You’re insane. You’re an otter, I’m a fish, end of story.” And it IS the end—the final scene (I think) is her swimming away with her family/friends to a new river/ocean/somewhere. The move has been discussed throughout the story, and the loner knows it’s coming. He sits on the riverbank and watches her swim away.
Main plot??: A predatory bird (eagle? hawk?) is threatening…something. Either the otter community, the fish, or some other furry/scaly thing that will die if the bird isn’t stopped. But this is like birdzilla, and nobody goes against it and lives. The loner would know—this same bird killed his mother and sister (I think?) and captured/tried-to-capture him when he was young. That’s how he lost his eye. So the bird is wrecking/intends-to-wreck havoc, and the loner tries to gather help to stop it (including the girl otter and her guy), but no one will help. So the loner goes off alone to face the impossible. Once the girl otter realizes this, she runs to help and finds the loner dazed/unconscious from a fall—with his eyepatch shifted up. That’s how she finds out about his eye. Also something about a nest—the bird’s or some other bird’s…pretty sure the loner fell from the nest or was pushed out and that’s how he passed out…or something? Hmmm. Can’t remember if they kill the bird or not, but there may be a slingshot involved and the day is definitely saved.
So…you can totally see what bits of the story fascinated middle grade me. Hmmm.
Books it’s not:
— Otter & Odder: A Love Story. This PB is the first thing that comes up on google. Definitely not it.
— Anything by Brian Jacques. I loved the Redwall books, but this wasn’t one of them. And according to wiki, Jacques didn’t write non-Redwall stuff until waaaaay later, mostly 2001+.

 Thanks for the help! I really appreciate it.