First of all if you want the amazing Maril from the future Game Boy Pokémon games you need to be playing the yellow version game.
Now, you need to have beaten the Elite Four 15 times (no more, no less) and you need fly and surf.
After you have beaten the Elite Four for your fifteenth time you need to fly to Misty's Gym. Then walk in and surf away from the shore inside.
Use a super rod in the middle of the water and you can battle a Maril.
