Find Sig in Underport was a mission in Jak II. Tess told Jak that Krew had sent Sig on one last mission into the Underport. Jak seemed to have an idea concerning the sudden appearances of Metal Heads in Haven City and decided to go and find Sig.
The PlayStation 3 and Vita versions of this mission are related to a trophy named "Finders Keepers", completing this mission will reward you with a bronze trophy.
The last and most difficult Titan Suit mission and a unique one at that, happens nearly entirely underwater. It is important to note you have limited oxygen, which can only be restored from underwater vents of sorts releasing large air bubbles into the water. Many explosive mines and dangerous new squid head enemies will also hamper your progress. Squid heads will latch onto you and electrocute you, dealing a huge amount of damage, you have to hit them at just the right distance and angle to kill them which is difficult as well with all the mines floating around. IF you can just avoid them you might want to do so.
For now just go through the fairly linear level, make sure to go past mines as fast as possible as waiting means they'll drift closer towards you. If you wait too long the gap in which you can go past them safely diminishes a lot, making it more likely for you to get hit. Eventually you'll reach a safe room with four metal claws holding a platform underwater, punch all of these to release the platforms which will cause them to rise.
Take the elevator up (you'll get out of the water for a short time) and use the platforms to get to the other side. Here you will have to submerge yourself again and enter this side-room with a gate. Now you'll have to make your way through a few more squid heads, go over a bridge and enter a room with a stone pillar. It will rise when you stand on it, so jump onto the next which does the same. Another jump and a short walk will take you to the airlock end this mission, where after you immediately go into the next; "Escort Sig in Underport".