well after you go through the encounter in mossdeep he is still in mossdeep in a house to the north western part of mossdeep
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Steven gives it to you in mossdeep city. You have to beat the mossdeep gym before you can use it. They are Psychic types so be careful.
In mossdeep city you have to go two houses at the left of the gym and you,ll find dive by steven.
You obtain HM Dive in Mossdeep City from Steven after getting the Mind Badge.
You talk to Mr. Stone in Rustboro City after delivering the letters to Champion Steven and Captain Stern.
It at the cutters house in Rustboro city(you have to defeat the gym leader Roxanne in Rustboro city to be able to use it).