Just got done catching one.
What you'll need:
-Repels
-Adrenaline Orbs
-Pokemon who can tank water moves
-That same Pokemon should have a very weak move or a status move
1. Once you get surf, head back to the first island and go to the Professor's Lab.
2. Pop a repel and surf down from the beach, and there will be three large rocks with a fishing spot in between them. It's not far from the beach, don't go too far.
3. If that fishing spot has ripples, fish there. If not, surf back up onto the beach and try again (this counts as moving between routes, resetting the fishing spots).
4. Keep fishing til you find a Corsola (or Mareanie). Remember to reset the ripples between each fishing trip.
5. Once you find a Corsola, pop your Adrenaline Orb and get it to below 50% health. It'll keep healing, but just try keeping it low. Once it's low, wait for it to call out for help. It's good to use status or stat-changing moves to burn through turns.
6. It'll take a while, but the Corsolla will either call more Corsollas (kill them) or a Mareanie. Be patient, and once one shows up kill the Corsolla and catch your Mareanie.
