this depends on the under water cavern you are refering to. there is one between pacifidlog and slateport througj the currents, just stay towards the bottom of the currents. there is also one on...
Go to Route 128 and find some dark spots in the water. From there, use Dive. It should look like underwater canyons that stretch out for awhile. Explore around until you find a cave entrance. Once...