this is in some sense the only thing I know for sure
You don’t. All your specific experiences are imprecise approximations: you can’t be sure what exact color you saw for how many nanoseconds, you can’t be sure all your brain except small part implementing only current thought haven’t evaporated microsecond ago. So you can have imprecise models of a fish brain the same way you have imprecise models of your brain—your awareness of your brain is casually connected to your brain the same way your thoughts can be casually connected to a fish brain. You just can’t be fully fish.
You don’t. All your specific experiences are imprecise approximations: you can’t be sure what exact color you saw for how many nanoseconds, you can’t be sure all your brain except small part implementing only current thought haven’t evaporated microsecond ago. So you can have imprecise models of a fish brain the same way you have imprecise models of your brain—your awareness of your brain is casually connected to your brain the same way your thoughts can be casually connected to a fish brain. You just can’t be fully fish.