Surf and Waterfall are the two HM moves that really don't need a buff. Waterfall is still a viable option for physically-inclined Water-types, competing only with Liquidation (which has a somewhat worse distribution, although becoming a TR move in Sw/Sh did change that a bit) and Aqua Tail, which has lower accuracy in exchange for slightly more power. Waterfall is still debatably better than either for fast Water-type attackers like Barraskewda, which can abuse its respectable Flinch chance to potentially rack up even more damage.
Surf, on the other hand, was the go-to Water-typed special attack for nearly any Pokemon that learned it all the way up to the 5th generation, when Scald was introduced. Scald is honestly overpowered and long overdue for a nerf, and were that to happen, Surf, which is by no means a bad move in its own right, would probably start seeing some usage again.
Everything else could probably use a buff if HMs ever return, though. If you're going to be forced to carry a move, that move shouldn't totally suck in battle.