Could you add fish entities to the Overworld and End? (not against adding them to the Nether but it's not as necessary since you can't really see through lava like water anyways) I think having them visible in the water would help people know if they are fishing in the right place and would help immersion.

Sorry, saw your original issue but forgot to respond! I think fish entities would be super cool, but they would take a lot of work to model. One possibility would be using the fish item models and expanding them to look like the typical fish models (might not look fantastic, but it would be better than nothing). I will look into it soon, thanks for the suggestion.