How fish arrive in a body of water

 Duck poop.

That's the short answer.  Bulgarian scientist proved this with an experiment.

Fish eggs can hatch even after being eaten and excreted by ducks

link

https://www.washingtonpost.com/health/fish-eggs-can-hatch-even-after-being-eaten-and-pooped-out-by-ducks/2020/07/02/700761e4-baf0-11ea-bdaf-a129f921026f_story.html

Pooping ducks can shed the live eggs of fish

link

https://www.sciencenewsforstudents.org/article/pooping-ducks-can-shed-the-live-eggs-of-fish

Fish eggs can hatch after being eaten and pooped out by ducks

link

https://www.sciencenews.org/article/fish-eggs-can-hatch-after-being-eaten-pooped-out-ducks