There will be 3 individuals sitting back on the cave walls, Captain Lockbard, Drew and RuneScape gold Cabin Boy Jeremy. Speak to Captain Lockbard and he'll thank you for finding him. He'll provide you 3 classic lamps he found in the cave. A cutscene will happen showing you bringing the 3 people up into the ship with the assistance of this Breathing Orb. Congratulations, quest complete!
To find fish pets you have to have level 73 summoning. Locate fish eggs in submerged caves or elsewhere, (should have level 73 summoning) if you find a shark pup, then you are extremely lucky. If you take more than 1 egg you can only get 1 fish. Proceed to the Fish Tamer's room of the palace and he'll have a fish egg incubator (you can't incubate any other egg compared to a fish egg in there.)
Put the egg inside and wait till you have a fish (you can not place the shark pup in it.) You may have (from rarest to least rarest) Sharks (rarest): They are the best helper in submerged fishing. They help you catch fish underwater quicker based on the stage. Stages are puppy (there isn't shark eggs), juvenile afterward mature. The next is that the Angler Fish. It's the sole source for light in dark underwater cave, as candles or lanterns would go out. They have three stages, egg, larvae and adult. You have to incubate the egg.
Pufferfish will be the next. Their eggs are found on the surface of the water that you can access from climbing up the anchor . Climb up it to go to Captain Murk. Get a pufferfish egg and then incubate it. It will hatch soon. The most common fish is the salmon. They help you find eggs. Whenever you are in 10 squares of eggs, then it will float in circles.
All fish have to stay submerged. You can't go from the water with it following you or in your inventory. You can have just one fish at one time even if the fish is with the tamer. Why you cannot put them in the lender is because you have to cheap OSRS gold give them into the Fish Tamer.