Get Kyogre..
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.
unova
i think you get to the underwater cavern after beating the 7th gym leader
MOSSDEEP CITY
Mossdeep City
beat the 8th gym
Swampert.
after you beat flannery you go back to where your dads gym is and beat that gym hope it helped :p
The gym leader in the normal gym, your character's father will give it to you after you beat him.
go back to your fathers gym and defeat him
After you beat Norman in Sapphire, head next door (the house to the left of the gym). There, Wally's parents will give you HM03 (Surf).
In Pokemon Sapphire, Ruby and Emerald the 7th Gym that's located in Mossdeep City has Psychic type Pokemon.