The goal of this quest is to dive at least 15 meters under the surface of an ocean. This task can be fairly difficult, as even in the deepest of oceans, it is difficult to have a gap of 15 meters between the surface and the player's head. However, one can be sure that the quest is complete once the chat says, "You have successfully dived deep into the ocean." You aren't finished yet, though, as the Sadhu also requires an ink sac to finish the quest.
Once the dive is complete and the ink sac has been gathered, return to the Sadhu. He will give you 128 reputation, 4 experience points and the ability to move onto the tenth quest in the series, Window in the Depth.