How do you get the boat ticket in Pokemon Emerald?

To obtain the Boat Ticket and access the Battle Frontier you will need to first beat the Pokémon League Champion, Wallace and defeat the Elite Four. When you arrive in Mossdeep City you will be given the option to take a ferry to an undisclosed location. Accept this offer. You will then land at the Battle Frontier and meet and talk with the Battle Frontier receptionist, who will give you the Boat Ticket at the end. The boat ticket will then become available in your Key Items pocket of the Bag and can be used at Lilycove City's port whenever you want to venture to the Battle Frontier.