Wfter dealing with the Gym, i went into the Spirit Mansion (basically Pokémon Mansion but way earlier) in Seafin City after fighting the rival, which was a total pushover, only having 2 mons, got a letter and continued south... only to find some water route that i can't traverse yet.
with no clue as to what to do next, i wandered for a bit (also a nice excuse to restock items and level up the rest of the team)
then realized that the letter i got earlier advances the plot (I had to give it to a scientist inside the museum in Seafin City), that's on my honestly.
Also found this thing, which had Wonder Guard and used Perish Song/Explosion on Nidorino. Poor thing.