Can also do ocean fishing, I saw ppl leveling there when I was looking for fishes for the achievement and they'd get like 3-4 levels just fishing and I think you get a bunch of xp at the end too. Can't chain it, it's only once every 2 hours but it seemed stupidly fast.
But yeah fishing is the one gathering skill you can level by just doing it at a decent pace, assuming you have manuals. Others you have to do collectables cause it's slow as shit, but fishes you get a bunch of xp for every fishes and with Patience you can just chain HQ so you always get big xp. You can still do collectables on top too in the right places, so it's really fast and easy to level.
Fishing is a huge completionist black hole though, on the surface it's easy enough but once you start the quest for big fishes this shit takes forever. Like mining/botanist to get the most current shiny tool achievement you need like 280 or 300 unique items or whatever, that's basically all of them, but it only takes a few hours to do, there's a few unspoiled nodes but most spawn twice an hour, a few of the older ones only once an hour but if you plan properly for it you can be done in a big day. Fishing on the flipside requires 1440 fishes, 200+ of which are big fishes. The fishes are random and some have low % bite rate, some only spawn at certain hours or during certain weather. The big fishes usually combine several of these conditions, on top of not being guaranteed catches when you get a bite(even old expansion ones, you can have 2000 gathering and ARR big fishes still have a set "miss" rate). Some are so rare they can go weeks without even being available to TRY. Real weeks, not ingame weeks.
It is pretty fun filling up that log though, there's quite a lot of depth to the fishing system once you understand some intricacies(and it keeps evolving, I learned about chum cancel last time I resubbed, a tech found because of how stupid some of the new big fishes are in ShB). But I wouldn't really recommend it, as you're probably looking at like 250-300hours of fishing to even get somewhere, which is hilarious when you compare it to the ~6hours for the other gathering.