>>42805107I run Sludge Wave Swampert and I find it infinitely more useful than Earthquake because Steel does not resist Water. Hydro Cannon is such a spammable attack that it's basically free damage. There are only two Steel-types I can think of that don't fear a STAB Hydro Cannon and one I've never seen in UL (Ferrothorn) and the other I've seen so little that it's a non-issue (Empoleon). Not to mention that no Steel-type will ever switch into Empoleon for fear of STAB Earthquake, so running Sludge Wave is a very safe bet.
Also, as
>>42805158 pointed out, bulky fairies like Clefable think they can tank a hit while they charge up a move only to find it's Sludge Wave and get deleted.
Sludge Wave also annihilates any Grass-type switching in that's weak to it. Even against Grass/Poisons, Sludge Wave does respectable damage and more than Hydro Cannon would. Earthquake will never save you in those matchups; if they switched in to your switch in you are dead anyway so you might as well do as much damage as you can while you're in there.
>>42805219pvpoke.com is a nice look at a glance at what's good, they also have resources to show you what learns what and what every pokemon's learnset is so you can help decide what's worth powering up or not.
gamepress.gg is also a good resource for information.
