u need a relicanth and a wailord and some pokemon that knows dig,surf,dive .then go to 132
and hit the ocean currents until u find a little spot of dark water then dive. there
should be some writing on wall somewhere that says go up here.then u go way in the back
where it says dig here. then the door should open and u go in the back and press on the
writing then u should hear doors opening. and tada!!!! (im not sure if this is completely
correct so look for more detailed answers)

regirock is in a small cave at bottom of the desert above mauville. registeel is another
cave far left and of lilycove city then go straight up and thetre should be a guy who is
standing next a rock. You have to use fly in the middle of the room to open the caveand
regice is on the route from rustboro to dewford in the sea. Hope this helped