Once you enter, go right and push the boulder into the hole using Strength.
Go to the other side (past the first ladder you see), and push the other boulder into the other hole.
Go back to the original hole, and fall into it.
Keep on pushing the boulder and following it (falling into the holes with the boulder) until you reach the water.
Once you do, the current will take you to another level.
Go up onto the land, left, and down all the way for an Ultra Ball. Take the only ladder to another level.
Follow the path south and you'll find a puzzle involving a few holes and plenty of boulders.
Push the only boulder that you can push left all the way left so that it doesn't block your way.
Next, push the single boulder into the hole. Now you'll have to work on the last two boulders.
Push the one on the right all the way up, and find a way to push the other boulder into the other hole.
Now jump into that hole. You'll reach another level. Surf up, and you'll meet Articuno.
After the encounter with Articuno, go onto land and take the ladder. Go down afterwards and take another ladder.
Go right and pick up the Pearl. Go right to take another ladder that'll lead you to a Water Stone.
Come back, go up the steps, and take the ladder.
Push the boulder that you've pushed in the beginning into holes and follow it. Do this until you reach water.
The current will be blocked, and you'll be able to go onto land.
Follow the path, and soon, you're out of the Seafoam Islands!