I know that there are no HMs in Pokémon Sun and Moon. That said, two of the Pokémon in my party do know Rock Smash. However, I've encountered multiple "sturdy-looking rocks" at this point and I haven't been able to get past any of them. The game just says:
It's a sturdy-looking rock. Maybe a powerful Pokémon could break it..."
What does it mean by powerful Pokémon, if the key isn't to use Rock Smash? Furthermore, how do I break sturdy-looking rocks in the water?