Once you have finished Deep Sea Troubles you can talk to Gompar for a different one called Shipwreck. Gompar will inform you a ship has sunk as it crashed into the walls of Mogrin. He'll want you to fix the wall. Go up to Captain Murk and tell him to sail closer to the wallsocket. He'll do so. You may then dive.
You will still be at the surface of the sea. Fix the wall. Go back up to Captain Murk and let him sail a bit further away so you can dip. He'll do that. Go to the palace (could be entered through some area ) and talk to Gompar.
Tell Gompar you mended the wall. He will thank you and give you 16,000 gp. Tell him you believe the boat belonged to a captain who was sailing this way. He will say he saw a few human beings swim to a nearby cave at an water cliff but it caved in due to a small earthquake. Proceed to the cave and mine the rocks blocking it (requires 43 mining.) Go in the cave and it will be dry in there because the water can not reach far enough to flood it.
Talk to Captain Lockbard and he'll thank you for discovering him. He will provide you 3 classic lamps he found in the cave. (Rub them for 10,000 XP in any ability.) A cutscene will happen revealing you bringing the 3 individuals up to the ship with the aid of this Breathing Orb. Congratulations, quest complete!
To get fish pets you must have level 73 summoning. Locate fish eggs in submerged caves or someplace, (must have level 73 summoning) if you find a shark puppy, then you are extremely lucky. If you take more than 1 egg you can just get one fish.