Get seal piece at Water Slums was a mission in Jak II. Brutter paid Jak back by finding out the location of a Seal piece for the Seal of Mar out at the Water Slums, however when Jak went to pick it up it turned out to be a trap by the Krimzon Guard.
Considered to be one of the most difficult missions in the game, there is actually numerous ways to do it. To start with, the water is guarded by a robot guard which will kill you in one shot if you attempt to swim away. The first strategy for this mission is to use a Dark Bomb and destroy it instead, where after you sail out smoothly on the JET-Board. However, you will need to make sure to preserve Dark Jak before the mission.
However, if you want to avoid the water you will have to fight your way past numerous guards and air trains mounted with a turret that will shoot you (and not miss) if you stand still for one second within its range. The second strategy also involves the jet board and this time you race past all the guards and air trains by going over the rickety scaffolding. Still difficult since slowing down (very easy considering the area is far from perfectly smooth) means you'll probably be blocked by guards.
Thirdly you can just run past all the guards, long jumps can easily give you enough distance and if you punch your way through the guards and citizens the air trains won't be able to hit you either. Lastly you can take the slow route; shoot down all the dropped off guards one by one with the Blaster and wait for the air train to leave where after you slowly continue.
Another simple way to deal with this missing is liberal use of the Vulcan Barrel. Use the Vulcan Barrel to mow down the KG as they drop out of their ships can keep you safe. Just keep an eye on your radar to avoid them sneaking up on you from behind. As long as you don't get too close to the ships or lose track of your surroundings, you'll be fine.
Another option is to continuously spam the Punch attack while moving; the punch thrusts Jak forward enough to avoid the shots from the air trains, allowing you to continuously advance towards the exit without having to fight, though you'll still likely get hit a few times.
The mission ends as soon as you reach the Slums district.