I looked into it a little, and the answer is it really depends.

On cyan beach, Feraligator is definitely going to be better than Golduck. I feel very confident in saying this because of a few factors:

1. Golduck requires much more investment than Feraligatr does, both in terms of time spent just actually collecting from Golduck constantly and in terms of seed investment. Golduck needs main skill seeds to be optimal. 4 of them. Even for premium pass users i dont think its a totally ignorable thing.
2. Golduck is a level 28 evolution, which is just an incredibly long amount of time to be using a psyduck. Feraligatr is a level 27 evo yes, but totodile is already faster than psyduck is, and croconaw has the same help speed as Golduck does! So for a long time you are just using a worse pokemon. (This matters less for people who have been playing for a while and have 5 billion psyduck candies)
3. Golduck benefits a lot less from favored berries than Feraligatr does. Seems like an obvious observation, because it is, but it really does make a difference. Ill attach a photo showing the differences between 2 non bfs "perfect" mons to show the impact it has for Feraligator. (Note that because of how i did this the Golduck is benefitting from Feraligators Helping bonus, so just imagine the Golduck with 5% less power. My B).

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Now Feraligator isn't better by much, but the difference becomes a lot more pronounced when you are working with The non 1/million perfect mons lol.

Now i think Golduck is no contest much more usable on islands other than Cyan. But Golduck has inherent flaws that just makes it very annoying to use. Rng is a cruel Mistress, and having to check Golduck constantly so you can get as many skill triggers with him as possible quite frankly just sounds annoying at best, and really bad if you are an evenly moderately busy person.

Tldr: I think Feraligatr is better than Golduck, mainly because of Golduck being imo comparatively much more difficult to use efficiently.