Update: I have completed 95% of the walkthrough! The other 5% is confirming that you can fish (using the rod) for Krabby, Psyduck, Goldeen, Poliwag, Horsea, and Slowpoke around Cerulean. I have seen none of them on Route 4 (right outside Cerulean) or Route 24 (right outside Cerulean near Pokemon Tech), finishing up Bill's Lighthouse and Cerulean Gym events.
You can find Slowpoke in the wild grass on Route 4 and you can steal all the others from Pokemon trainers along the way, but I haven't found Poliwag at all. And from what I can tell, there's no space in Cerulean City proper where you can fish. So, "around Cerulean" is presenting its challenges for the Walkthrough's purposes of confirming certain pokemon. Does Val (or anyone else) remember which of the two spots (Route 4 or Route 24) is the one that has the other wild Pokemon for fishing purposes? This way, I can focus and fish in that particular area until I get lucky lol