So, you can still level fishing if you've unlocked fishing for that zone. One of my 19s had 306 fishing - that breaks down into 300/300 classic and 6/76 outlands. Fishing in the outlands raises the outlands skill.
So it sounds like you'll be able to level fishing up as long as you have that fishing unlocked. So even if you can't or don't want to get to 525 fishing, at least head to legion dal and unlock fishing up to 800.
I'd be interested to know if you have level 1 fishing, unlock legion fishing up to 800...then when can you use the raft? You have 1/75 panda fishing (=526)...so is that good enough? Or if you level cata fishing to 75/75 (=525) is that good enough? Or do you need to level everything to max up to and including cata fishing?