Obligatory epistemic status: Speculative and hard to test but falls out an as implication under current worldmodel.
Just a shower thought I've had on my mind for a while:
Fish would be bad swimming instructors (for humans).
This remains true even after adjusting for/overcoming the differences in body types and ability to speak your language.
What do I mean? Well, to be sure, fish are good at swimming. And normally, that's what's you want in a swimming instructor...
...but not here. While fish are good at swimming, they have always been good at swimming. They were deft swimmers before they reached any point of being able to reflect on their selves or what they are doing. They have no appreciation for what it's like not to be a good swimmer.
What does "learning swimming" involve (for humans)?
Fish, being fish, never had to make the transition from being a natural biped to one that has repurposed its body for motion in water, which is what humans have to go through to "learn swimming". They never had to start from a mentality that finds walking somewhat natural -- and swimming somewhat unnatural -- and then adjusts its appendage motion in a way more suited to the latter.
We can imagine any kind of instruction session between fish and humans (as above, appropriately adjusted for language) to be fraught with peril. The fish might try to point out deficiencies that hold back the human. But it has never had to think about such deficiencies, because it has never needed to distinguish between the right way and the wrong way. It was always just "the way" that came instinctually.
The fish might nearly lash out, frustrated that the human attempts an obviously ineffective means of water locomotion. But it has no idea why those flawed attempts are wrong. They just feel wrong.
The fish will happily demonstrate the "right way", just by doing it. But identifying the difference between its "right way" and "what humans do" would be a learning experience in itself, something it has no natural advantage in, as it was never part of the fish's "learning process for swimming".
The converse is true -- humans would be bad at teaching fish to walk (with the appropriate apparatus) or otherwise move on land. (But note the parallel isn't perfect as humans generally do have to struggle and learn how to walk, but not in any way that involves formal instruction.)
Implications for human-human interaction
So far, this insight feels (and, well, is) just idle speculation. But there are implications for ordinary life.
There will be times when someone is a "natural" at some skill. They're good at it. And that is their only teaching qualification. And they try to teach a non-natural that skill. They are then bad at teaching. The missing piece is the learner's mind is very different from what the natural is capable of filling in. Such a teacher will constantly show them how to do it "right" but not be able to identify the difference between what they're doing and what the learner is -- except by drawing on some kind of unrelated, general intelligence.
(I won't give any specific examples of such a skill, because those become contentious issues in their own right and detract from the general point. But I've definitely been on both sides -- learning from someone who has no understanding why they're good at it, and grasping to communicate something I do without ever thinking about it.)
You can also run into this problem when you become skilled at something. You can sometimes assimilate the skill so well that you are effectively a natural, by forgetting the whole process by which you learned it. You may lose the perspective you had as a beginner and are no longer able to relate to them.
Part of why I enjoy teaching what I know is that I seem unusually resistant to this process, and have vivid memories of the hurdles I overcame as a newbie.