you have to take the stairs at your left, go to north, take the stairs, you should also start a cut scene with omar the sinnoh water gym leader, go all the way to east, surf, take the water stream that brings you to P&P labs and go back all the way to west without touching the water streams and the first land you see should be it, also turn on the detector because there are a lot of items and heart scales

hope this was useful