Mossdeep City is a city in Hoenn. Tate and Liza (7th) are the Gym Leaders here. In R/S/E, the rocket launch center is taken over by Team Magma for rocket fuel later in the storyline. It is also apparent that Steven lives here. HM Surf is needed to get here and you will be able to use HM Dive from beating the Gym Leaders here.
In Pokemon Ruby and Sapphire, there's a house at the left side of the city(which a "Secret Base" finding kid lives in). The child's mother has a Skitty but when you interact with it, the dialog says "DELCATTY" instead of "SKITTY". This error was fixed in Pokemon Emerald.
In Pokemon Ruby and Sapphire, there is a man with a locked door in his house (the one below the Pokemon Center?) and he'll say "Everyone these days has a cable to battle. Nobody will battle with me. Now all I can do is watch." If you hack the game and open the door, it will take you to his basement and a replica of the player of the opposing gender is present. If you talk to it, it will say !!!!!!! !!!!!!! !!!!!!! and your game will freeze. In Pokemon Emerald, his home in Mossdeep City is replaced with the Mossdeep Game Corner and his home is moved to Sootopolis City.