From Halopedia, the Halo wiki
Rising Waters is an achievement in Halo: The Master Chief Collection. It is awarded for activating the first Halo 3 Terminal on The Ark. The achievement is worth 5 Gamerscore and can be unlocked on any difficulty. The player must scroll through and access both parts of the terminal to unlock the achievement.
After fighting the first set of Hunters, it is in the building in the cliff wall. There is a corridor with two holographic displays on the right, and two doors on the left. Through the doors is a semi-circular room containing the first of the seven Terminals.