I need help in the under water cave of pokemon emerald?

I was underwater, when i entered this little pageway which led me to a submarine. It said that it was where team aqua surfaced. So, i go into the cave itself, & theres a big block of dirt, or something, in front of me. To the left & the right, there are boulders, one is blocking an entryway & the other one seems to be random. I don't know how I'm supposed to get past through to the little "path".