you have to go to route 127 and use dive(basically in between mossdeep and sootopolis)

go to route 128 and you will see a big long piece of dark blue water after surf around a
bitt use dive look around for a cave entrance you will see a submerine go up get through
all of the boulders and crushable rocks {you must have hm06,hm04,hm02}you have to Archie
aka team aqua's leader then fly to sooptopolis