Seafloor cavern in route 128
go get a life
you first go to steven to get dive and dive dvie
he goes to his underwater hideout cave.
in your moms belly
Go to the submarine and press B. You'll notice you can get up there. If you are above, you'll need to explore a cave with team aqua in it.
Go back to Slateport town. Inside the ferry building, you will find captain stern and the submarine being robbed by Team Aqua.
Catch the submarine Pokémon and beat the crap out of the Elite Four.
you have to dive underwater by sootopolis city and you have to go in the little cave underwater where youll find the sud and then you surface there.
it is in team aqua's base (the one where they had the submarine)
In Slateport city harbour you will see capt. stern being interviewed with a large crowd talk to capt. stern and you will go into the harbour thing and then go to their hideout in lilycove city.
The submarine belongs to that scientist guy, you're not allowed on, you have to learn dive then find the place they are exploring, but there is a scale model of the sub in the museum.
in the cave where team magma escapes you in the submarine next to lilycove city