Keep checking the cave until the tide is down.
if you keep looking every day the cave will dry up and the white piles on the floor will be shoal salt hope this helps pokefans OK listen to him hes a genius I'm going to do that thx
go to homie brians house and open the refrigerator
deep inside shoal cave which is north from mossdeep city
go to mossdeep and surf up till you see a cave