This guide is for the trophy Iron Lung, which requires you to stay underwater for a consecutive minute without coming up for air.
As far as I know, the only place to do this is in the same water tunnel you get the wheel for the Pack-a-Punch as well as the plant for the KT-4.
Simply go under water and swim around. Within the tunnels will be 3 orange spores, if you shoot or bump up against them they will explode and give you oxygen. Make sure your character begins to gag for air and then pop one, do the same for the next 2 and your trophy will pop after a minute has passed without surfacing for air.
Note: One of the orange spores is at the very very far end of the tunnel immediately beside where you grab the plant piece.