A Level 25 Marill can still evolve into Azumarill even despite the fact that Marill evolves into Azumarill at Level 18, all you have to do is level it up and it'll evolve into Azumarill as long as it isn't holding an Everstone.
evolve a marril --EDIT: September 1, 2010-- Evolving a Marill would result in an Azumarill, NOT an Azurill. To get an Azurill, you need to give a Marill/Azumarill a SEA INCENSE from the top of Mt. Pyre, and put it in the Daycare with an Azumarill/Marill of the opposite sex or a Ditto.
Azuril evolves when friendship is high enough, and Marill evolves at Lv 20 to Azumarill. You catch Azuril in Trophy Garden after getting the national pokedex. Talk to Mr Backlot and tell him you will survey him. He will tell you a special Pokemon in the garden. To catch Marill, go to Route 215.