you have to beat the lilycove team aqua base
You have to surf there first. From Lilycove City, you surf to the right till you reach Mossdeep City.
Surf behind Mossdeep city and it will be there.
you have to surf from Lincoln .
You have to surf from LILYCOVE city to the right
Surf east from lilycove city.
Surf south until you can't surf no more, then surf west to Pacifidog.
look in mossdeep city
you have to start from mossdeep city then surf down and go right
To get to Mossdeep City, you have to defeat the other six Gym Leaders, and you need Surf. In Mossdeep, there is a Psychic-type Gym, and Team Magma will be in the Rocket place.
if you have the HM FLY then you can fly there or you can surf but by some repels if you are going to surf
Surf east to mossdeep city.
Get to Lilycove City, and once you defeat the Aqua hideout just to the northeast of the city, the wailmer will move, and you can surf east to Mossdeep.