From Bulbapedia, the community-driven Pokémon encyclopedia.
Sevii Isles 8 and 9 are locations which are not accessible through normal gameplay in Pokémon FireRed and LeafGreen. However, unlike other similarly-named unreachable areas, they have actual map data.
No Pokémon are programmed into the areas by default; it seems as if there are abandoned areas left over from the beta of FireRed and LeafGreen. No index pointers lead to it by default either; on Isle 8 the central island of the "route" has what one would assume to have been cave entrances changed into the side of a rock.
If it is entered when walking on land, it has the same route theme as the later Sevii Islands routes.