Lilycove is another pretty city:
https://m.youtube.com/watch?v=s2ut8s2hdSc To note there's a nice contest hall, art museum (do well enough in contests and you can get your portrait on the walls as well as a nice item for your secret base - a Glass Ornament) boating dock, move deleter, and a large department store (tutor for Substitute is found here, and look out for sales on TV if you still have a functioning in-game clock!).
We have to go back to Slateport before progressing here first, where Team... Aqua, again, are up to no good and stealing a submarine. They take it to their hideout east of Lilycove. Inside we have to once again take care of the baddies, but there's a Master Ball waiting for us as well.
Clear those chumps out and head east toward Mossdeep City.
