In the first room you first drop into, look for a switch on the rear side of the columns. A highly useful room opens.

Jump in the crescent pool. Just before the long corridor to the half-flooded room, get up and you will find a goodie room.

Howto get that lovely red armour... Well, first get to this beam. Shoot the big roof light, and the beam you stand on elevates.

Secret underwater room! Just shoot the different-coloured wall.