Instructions
Head to Mossdeep City. Once there, heal your Pokemon. Go to the house located on the northwest corner of the city. Ask the man inside for the HM-08 dive. Leave the home.
Locate the "Poke-mart." Head east from the entrance of the store until you reach a set of stairs. Head west from the stairs until you reach a woman. Head north from the woman and walk until you reach a house. Enter the house and talk to the boy, he will give you the fishing pole required to catch Relicanth.
Leave Mossdeep City heading south. Enter Sootopolis City. Locate Route 124. Go underwater using your HM-08 dive equipment. Head south underwater through the cave tunnel.
Look through all the seaweeds until you locate Relicanth. Unleash on your active Pokemon. When prompted, choose "Bag" from the options. Choose the Master Ball to catch Relicanth.