Apparently Waterfall’s not a HM in this game. Can’t it at least be a TM? Waterfall’s better than Aqua Tail in my opinion because despite being slightly weaker, it’s more accurate and has a potential flinching side effect. Plus Poliwrath doesn’t have a tail and it’s Special Attack, even in its Mega Form, isn’t strong enough for its Special Water Type moves to be a significant threat unless the opponent is weak to them.
They prob forgot to add it in, I’ve seen many complaints about it.