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Now, you know those two long platforms with a door at the end? (You see them when you first enter the main room.) The first one has the silver key. The secret here is that the water level must be DOWN. Go through all these platforms and jumps, and you will eventually come to a room with a pool in the middle, a door ahead and some platforms above. Do the regular platform-hopping and you will come to a switch that opens the door containing the silver key. Run down and grab it, and then you will find Pierre to nab you with his Magnum fire.
(Incidentally, the golden key is in the next room accessed by a rusty key (the second of the two long platforms). The secret here is that the water level must be UP.)