you have to go Surfing in the route to get to mossdeep then find a diving spot and use dive. then hope you find the entrance to sootpolis.
Use dive you will end arrive in sootopolis to see groudon and kyogre fighting, you will then have to find rayquaza in sky pillar
get rayquaza
You get Dive from steven not may.
you can find dive balls at the mossdeep city pokemart
You have to have a poke'mon with dive.
Dive:Mossdeep CityWaterfall:Cave Of Origin
to get a heart dive
find it using dive
Use dive.
You have to have a Pokemon in your party which know the HM Dive. Surf onto water then go to your party, select the Pokemon that knows dive, and click on Dive.
get a water Pokemon yhe TM dive and well dive under the water
dive 126_128 routes
mossdeep city dhoom