Drain sewers to find statue was a mission in Jak II. By the request of a client, Krew asked Jak and Daxter to return to the Haven sewers and recover the ruby key embedded in a statue of Mar Krew once lost while smuggling due to a flood. This mission gains a double nature when Krew uses the ruby key that Jak found to trick Sig into allowing Metal Heads to access Haven City.
Walkthrough[]
Another JET-Board mission, this time in the Sewers. Once inside take a right and head forward towards the water (notably inhabited by two znorkle tooths so avoid entering it) and use the JET-Board to enter the new tunnel on the right here. Go through it and at the end, hit the valve which will drain the water revealing a new tunnel to go through.
This time there's multiple mines as well as dangerous machinery; move past them (if need be slowly) and at the end there's another valve. This time you'll drain most of the water out of the tunnel, revealing another set of machinery that pops out of the ground. The trick to these is that the inactive ones become active when you get close enough, while the active ones go inactive instead (it does not appear to be related to a timer).
Past some more mines and a set of corkscrew-like beams you need to jump over is the third and last valve. Now the mostly drained sewers will lead more or less directly to the statue you needed to find; there's mostly just a lot of mines. At the end is a platform that takes you back to the area where you started.
Note that the events of this mission permanently drains the water from the Sewers for later visits.