Go to route 221, it's the one from the town where you got your pokedex. All you have do is swim to the pal park and
it's one ace trainer girl. She has Cherrim, loppuny and marill.
The Ace Trainer's name is 'Ace Trainer Shannon'
She is found standing in the grass south of 4 berry plants on route 221.
Otherwise you can evolve cherims