Bank any pets or summoning pouches you have on you, (He won't let you dive carrying any animal or summoning pouch, something to do with animal cruelty...) and head over to Murphy on the docks of Port Khazard, take a charter ship from Catherby for 1600 gold for the quickest route, or a charter ship from Port Sarim for 1280 gold. The easiest way to arrive is to use the Yanille Lodestone, which must be activated before use, and then running northeast from the eastern entrance to the city. It is also an option to use the spirit tree network to take you to the Khazard Battlefield, although this requires a slightly longer run to Port Khazard. Then ask him about giant crabs. He will tell you he can create breathing gear if you have a fishbowl. He can then take you diving for crab and kelp.
A mudskipper.
With a fishbowl in your inventory, talk to Murphy again to get your fishbowl made into a diving apparatus. Put on the fishbowl helmet and diving apparatus, and then ask to go diving. Your weight must be under 27 kilograms (diving apparatus and fishbowl helmet weigh 15 kg together, leaving 12 kg for your items) (there's a bank deposit box near Murphy), but keep your needle and bronze wire. Boots of lightness from the Temple of Ikov Quest may be used to lower your weight further, along with Wicked robes top and legs. Spotted and Spottier capes cannot be equipped as the diving apparatus must be worn.
For lower-levelled players, food and a weapon are essential, as the mudskippers hit fairly hard for their level. Note that capes and helmets cannot be equipped underwater.
When you are ready, tell Murphy you are ready to go diving.
Note: There appears to be a glitch where if you have an aura activated when you enter and it depletes while you are swimming, you will start to walk/run rather than swim. This can also happen if you log out and then back in while underwater.
Once you're underwater, find a tall green plant—kelp—and pick it. Then swim north to a crab pen. Once, there, you will see Mogre Guards. Talk to the one named Nung. Tell him you want one of the big crabs, and he will tell you to get him five mudskipper hides.
Swim west to the entrance to the Mudskippers' cave, on the western wall of the underwater area. Pick up five rocks near the underwater cavern entrance, and then enter the cavern. While in the cavern (and the crab pen thereafter), players will be able to wield weapons as normal, as you will be standing upright. Kill five mudskippers (level 48 and 50) to get hides (note: you can use Ring of recoil, but you need to have dealt at least 1 damage).
Exit the cavern, and take the hides back to Nung. Talk to him, give him the needle and three bronze wires, and then open the pen door. Kill a crab to get the Crab meat. (Be sure to bring the needle and the wire with you: It is not possible to take the mudskipper hides above the surface of the water!) Players may also want to get extra meat because you may burn the food when prepared later.
Leave the pen, swim south, and climb up the anchor (look for the little anchor). You may teleport from inside the crab pen if you prefer.
Right-click to grind raw cod into Ground cod (Talk to Cook first or else the option to do so will be locked in the cooking menu).
Use your knife on your bread to get Breadcrumbs (Talk to Cook first or else the option to do so will be locked in the cooking menu).'
Now talk to the cook again. Make your way through the conversation to the Protecting the Pirate option. The cook will tell you he now knows how to make fishcakes.
Use the ingredients on each other to create a raw fishcake, and cook it to get a cooked fishcake. Enter the banquet hall, and give (do not eat it!) the fishcake to Pirate Pete in the dining room for your reward.