I’ve spent days, almost a solid week, trying to find a solution for this and finally I’ve FOUND THE SOLUTION!
There’s a great addon called “Plumber” that has a module that allows you to see your artifact skill tree for the Underlight Angler. It’s called “Blitz Fix: Underlight Angler” in the addons options menu under Plumber. It should already be ticked.
So, it does work, but it’s wasn’t straightforward for me, still. You have to go to your forge in your Legion class hall, unequip your Underlight Angler and place it in your inventory, equip your Legion class Hall artifact weapon, click your forge like you are wanting to upgrade your Class Hall weapon, click out of the menu. Now, shift+right click your Underlight Angler in your inventory, then you can start upgrading. I had to do that every time I’ve revisited the forge after getting artifact power to upgrade a new skill, which was 4 or 5 separate times going to the forge. I’m not sure if the /run command you commonly find on Reddit or the Blizz forums initiated me seeing the first skill, but it didn’t before I downloaded Plumber and the run commands never bought a skill for me to begin with.
With that said, I’m assuming you have to have your class hall questline done or at least up to the point where you can start customizing your main class hall weapon on your forge. Which would make sense, since you’re trying to upgrade a legion artifact.
These may not be exact steps, but this is just my experience while working with the addon and the broken tree we are given.
I now have the waterwalking ability and underwater swim form thanks to this addon. I can not thank the developer enough.
Also, this may or may not be related, but before filling my artifact tree out to swim and waterwalk, I had all of my profession equipment hidden, so that I do not see it transmogged on my character after use. After getting the swim form/waterwalking, I could not use either. I relogged and reloaded, still nothing. I went back to the NPC to enable showing my fishing equipment after use and was instantly able to use both skills immediately after doing /reloadui and finding the nearest body of water.
TLDR; Download the “Plumber” addon, go to your forge, unequip your fishing pole and place it in your inventory, equip your artifact WEAPON, select the forge, close out of the menu, shift+right click your Underlight Angler in your inventory, upgrade your skill tree accordingly.