Bring the Southsea Treasure back to Wrenix the Wretched in Ratchet. Do not forget to get an E.C.A.C. and Thieves' Tools from Wrenix's Gizmotronic Apparatus. You will need both of these items to complete your mission. Should you be attacked by any unusually hostile parrots, use your E.C.A.C.!
I don't mean to scare you, [player], but the last five thieves I sent to steal the southsea treasure came back to me in pieces.
South along the coast you'll find Tide Razor, the southsea freebooter's ship. Nestled in the hold is the payoff - the southsea treasure. On the second level of that ship is the pirate's den, a place where they train all of their thieves.
Board that ship, steal the treasure, and bring it back to me. Use the pirate's den if you need to practice your thievery, but be careful!