you swim to the fountain near where the girl who gives you the mirror thing and where the fountain water goes into the sea and and hover your mouse round the circle that it splashes and it would say "dive"
you dive down underwater and you go into a cave
swim of shore and there it will say dive thats the underwater cave
Use Dive.
marine cave is found when you dive then you dive and look for a cave, it starts out as a a normal cave then turns into a misty one
Just dive by the circle of darker water and you will enter the underwater maze.
scuba diving
You have to dive underwater and find a submarine.
go to route 127. use dive. it will be there.
In the underwater cave, use dive (B button) to come to the surface. You end up in Sootoplits City. Hope this was helpful ;)
the entrance is underwater you have to complete the phsychic gym to learn dive and dive near sootpolis island then you will find a path that leads to a sort of underwater cave and then click dive again to go up
Don't you mean "How to get to Sootoplis City in Pokemon Ruby?" Oh it's simple, get dive and dive underwater near Mosdeep City and find the path, then keep going down until you find a cave, go in the underwater cave and then dive again and there you are!
You click the darker water underneath the statue of Poseidon on the seahorse.