Reach Port via sewer is a mission in Jak 3. Samos Hagai advised Jak to go into the Haven sewers and exit into the Port district, from where he would need to head north and find a way into the New Haven district where he could re-unite with him and Keira Hagai.
The PlayStation 3 and Vita versions of this mission are related to a trophy named "Swamp Gas", completing this mission will reward you with a bronze trophy.
First you will be attacked by wasps which are easily dispatched with the Beam Reflexor (an all-around excellent weapon in the Sewers due to the tight rooms). The Sewers are a pretty linear level, with only a few hidden side rooms indicated by large vents on the floor, use the JET-Board on top of these to gain a jump boost and enter a hidden room. Continue until you reach some sewer frogs, easy prey to the Beam Reflexor or the Wave Concussor.
Another jet board section, take care not to fall into the depths at the end of the tunnels, you will soon enough reach a new area flooded with water and more sewer frogs as well as saw fish. Saw fish aren't that fast on land and are easy prey to the Reflexor and Concussor again. Up the sloped stairs (using the jet board) you will find three hard to see needle fish in the water, avoid touching these as they explode and fire off needles in their immediate surrounding. Use the jet board to avoid them with ease, there's one of the hidden rooms right here as well.
Cross the grind rail and equip the new Arc Wielder to sweep the floor ahead of you clear of needle fish as well as the large fans blocking the corridor. There's more needle fish here as you go up the slopes using the jet board, at the top you will encounter a whirlpool, go around and take the water slide on the right. At the bottom you come to a spacious room with two more whirlpools and numerous sewer frogs and a couple of saw fishes, all easy prey to one or two charged Wave Concussor blasts.
Kill them all to somehow cause a grind rail to fall down allowing you to move onwards. There's more whirlpools here as you use the jet board to go up the slopes, there is another set of fans here as well. Entering the next room you have to go up another slope to encounter two juice goons, sturdy Metal Heads with a short range taser staff. They're not a match for any of your weapons at this point, so dispatch of them and cross the nearby metal bridge and head up the slope. Another batch of sewer frogs here and after these the second-last stretch to the exit, filled with more juice goons and sling blasters.
The Reflexor works well here though the Vulcan Fury or Arc Wielder are effective too, just avoid staying still if they do manage to fire a couple shots at you. The last room has a couple more wasps, where after the way to the exit is clear.