Set the egg RS gold inside and wait till you have a fish (you can't place the shark pup in it.) You may have (from rarest to least rarest) Sharks (rarest): They are the ideal helper in submerged fishing. They allow you to catch fish underwater faster depending on the stage. Stages are pup (there isn't shark eggs), juvenile then adult. The next is the Angler Fish. It is the sole resource for lighting in dark underwater cave, as candles or lanterns would obviously go out. They have three phases, egg, larvae and then adult.
Pufferfish will be the next. Their eggs are found on the surface of the water which you can get from climbing up the anchor . Climb up it to go to Captain Murk. Get a pufferfish egg and incubate it. It will hatch shortly. The most common fish would be the salmon. They help you find eggs. When you are in 10 squares of eggs, then it will float in circles.
All fish have to stay underwater. You can't go from the water following you or in your inventory. You can only have one fish at one time even when the fish is using the tamer. You cannot go out of this water with fish (besides eggs). Why you cannot put them in the lender either is since you have to give them to the Fish Tamer.
You can also have a house underwater in Mogrin. Your dungeon will be temporarily removed until you move your house to a location that's not underwater. Having a house in Mogrin unlocks a brand new attribute for your PoH. Whenever you're in Mogrin you can add an aquarium to your PoH. When you move your property, it will still be there and you'll be able to upgrade your aquarium items but you can't create another aquarium until you return to Mogrin.
To make an aquarium, then you must have level 54 construction. When you put in your aquarium, then you will understand a hotspot which resembles a massive fish tank. To create it you must have level 54 construction, 3 regular boards, 8 nails and 4 molten glass bits. This way you can have 5 little fish or a single medium sized fish. For them, visit the Fish Tamer. Sorry people, crisis, gotta go, I will update more buy OSRS gold later.