It really depends on what you mean by "regional fish Pokemon". Some generations have multiple Pokemon based on a fish, and I would argue that Goldeen/Seaking (for example) are even more the regional fish Pokemon than Magikarp/Gyarados.
Here are all the Pokemon that are clearly based on fish. I have only included Pokemon that follow the standard fish body type, so there will be no Skrelp or anything here, although they can be argued to be a kind-of fish. The one in Bold is the evolution line I consider to most be the "regional fish Pokemon."
- Goldeen --> Seaking
- Magikarp --> Gyarados
- Chinchou --> Lanturn
- Remoraid --> Octillery
- Carvanha --> Sharpedo
- Barboach --> Whiscash (I didn't consider this to be the regional fish, simply because I find them completely forgettable)
- Feebas --> Milotic (Since Miltoic is not really a fish, I again didn't consider it to be the best regional fish Pokemon)
Kalos doesn't really have one (Skrelp/Dragalge are probably the closest).
Source: Just looked over the National Dex.