EDIT: There seems to be a delay for this achievement to pop. Please allow an overnight delay irl before expecting this achievement to unlock.
Very easy and unmissable achievement.
Play on Survival, Freedom or Hardcore Mode.
After the opening cutscene has ended you will regain consciousness to find your Lifepod is on fire. Pick up the fire extinguisher off the floor and use RT towards the flames to extinguish them. Your fire extinguisher has a limited usage so don’t be wasteful with it.
Now you can go underwater for the very first time, and this achievement will unlock.
To go underwater you’ll need to exit your Lifepod. You can exit your Lifepod two ways.
You can interact with the ladder in the middle of the Lifepod to climb up it and exit through the door at the top of the Lifepod. You will then need to dive down into the water.
Or you can interact with the door on the floor to exit through the bottom of the Lifepod. You will immediately be underwater by exiting this way.
Please Note: I was playing on Freedom Mode (similar to Survival Mode, but without the requirement to maintain hunger and hydration levels).
Currently I am not sure whether all achievements can be obtained in Freedom mode.