Put the egg in it and wait until you have a fish

[Maymay]

Tell Gompar RS Gold you mended the wall. He will thank you and give you 16,000 gp. Tell him you believe the ship belonged to a captain who was sailing this manner. He'll state he saw a couple human beings swim to a nearby cave at an water cliff but it caved in because of little earthquake. Proceed to the cave and mine the stones blocking it (requires 43 mining.) Go in the cave and it will be dry in there since the water can not reach far enough to flood it.

There will be 3 individuals sitting down on the cave walls, Captain Lockbard, Drew and Cabin Boy Jeremy. Speak to Captain Lockbard and he will thank you for finding him. He'll provide you 3 antique lamps he discovered in the cave. A cutscene will occur revealing you bringing the 3 people up into the ship with the aid of this Breathing Orb. Congratulations, pursuit 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 discover a shark puppy, then you are very fortunate. 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 will have a fish egg incubator (you can't incubate any other egg than a fish egg in there.)

Put the egg in it and wait until you have a fish (you can't put the shark puppy in it.) They help you catch fish submerged quicker depending on the stage. Stages are pup (there is not shark eggs), juvenile then mature. The next is the Angler Fish. It is the only source for light in dark underwater cave, as Buy OSRS Gold candles or lanterns would obviously go out. They have three stages, egg, larvae and then adult.