Swim on the other side to reach a wide corridor, there's a Praetor Suit point after the passage. You'll find a Codex Page and a Toy blocked by a grid. To open it, you'll have to break the nearby statue and activate the green button, then climb on the shiny pedestal.
As you walk up to the large circular room, you'll get a cutscene. Remember to look underneath for extra life.
In the next room, 2 targets will have to be punched within a limited time to open the hatch. Make good use of the horizontal bars and the dash.
Go down into the sewers, you'll come across the Slayer Gate. There follows a puzzle room where you'll to purge the water by dashing underwater through the grids, then climb up on crates. You also have to activate targets in the holes by shooting at them.
Just before leaving the sewers, turn left for the Slayer Key, this will open a gate allowing you to reach the door.
In the next room a big fight awaits you, as well as BFG-9000 ammunition. Position yourself where indicated on the map to find a destructible wall. This will unlock a secret as well as an optional fight against a Marauder.
