You have to go to the pal-park south of Sandgem, it is at the end of the L shaped route, trainers and clumps of tall grass are scattered throughout. Once there at the park, you'd have to select 6 Pokemon and catch them in a multi-terrain park. Surf is required to catch water-prone Pokemon like Seal or Clampearl. Pokemon that know HMs aren't allowed to transfer (different HM sets between the two games). Once you caught the 6 chosen Pokemon, they will be moved to your PC system. There are points given to you after you finish. These points are irrelevant if you're not intending to compete with friends or yourself.
You will need a Pokemon that knows surf to get there.
In order to transfer Pokemon from your sapphire, you must complete the national Pokedex before you can do this. If you did, no need to comment.