Pokémon FireRed and LeafGreen/Seafoam Islands

You can't get through Seafoam Island unless you have a Pokémon that knows Strength

After you enter the cave, go east and push the boulder into the hole to the north. Now go to the steps north of the entrance and go up. Grab the Ice Heal.

Also you can catch the legendary Pokémon. When battling it, try to use techniques such as paralysis and sleep.