As I pass 7100 cast I was wondering if I am trying the correct place. I am on the dock in Zandalar across from the quest / table boat in the dip near the other boat. It says Port of Zandalar zone and I am catching Slimy Mackerels. Am I in the wrong place or did I use up my luck completing Mecha Done and Undersea Usurper?
Hope for others, caught it at 8611 attempts in the bay.
No, that location should be fine.
It’s just that they massively cut the drop rate on it.
7100 does feel like the far edge of the RNG, though.
You are perfectly fine where you are fishing for this particular mount as I’ve seen players get it on the docks where you described.
However, Blizzard did nerf the drop rate quite some time ago as it used to be a high rate where it would drop for players. As for what the drop rate is now, I’ve got no clue. But I know it’s a lot lower than what it was.
Hopefully you get this mount soon to earn the achievement for it. I wish you the best of luck! 
Thanks for confirming the spot. Only got trying to catch the ray, stockpiling doubloons and leveling alts for the Heritage achieves till 8.3 anyway.