MOSSDEEP CITY
Get Kyogre..
Mossdeep City
In Pokemon Sapphire, Ruby and Emerald the 7th Gym that's located in Mossdeep City has Psychic type Pokemon.
To defeat or "beat" the 7th gym leader in Pokemon sapphire there twins and they have a solrock and a lunatone to beat them I suggest about level 40-50 water types.
In Mossdeep city. On the right side of the island. (On the 7th gym island)
it is a dragon type Pokemon gym
The third gym leader in Pokemon Sapphire is Wattson and he is located in Mauville City.
Pokemon platinum is for DS but it is in snowpoint
fire gym in lavaridge
the seventh gym in pokemon sapphire is in mossdeep city.
In sapphire, the 5th gym leader is your dad, in that city after Oakdale.
i think you get to the underwater cavern after beating the 7th gym leader