PokéBase - Pokémon Q&A
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Please fix your tag. It should be Emerald not SoulSilver

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It can only be accessed after downloading the MysticTicket through
Mystery Gift and sailing there by talking to the sailor in front of
the Seagallop Ferries in Pokémon FireRed and LeafGreen, or by boarding
the S.S. Tidal in Pokémon Emerald.

See: http://bulbapedia.bulbagarden.net/wiki/Navel_Rock

If you do not have the Mystic Ticket already you will not be able to obtain it without an Action Replay.