The American Humane Association rated this film "unacceptable" because of the rat that was submerged in oxygenated liquid in one scene.
"James Cameron later admitted that four rats had indeed gone through the procedure without problems; the fifth, however, suffered a cardiac arrest. Fortunately, Cameron was able to revive it through careful chest compressions, and later kept it as a pet."
Supposedly, the only purpose for the cuts in the sequence was to avoid showing the rats defecating from panic.
And you need only watch the scene to see a rat in enormous distress.
There is a scene near the beginning where a white rat is used as a demonstration for a liquid oxygen. Her owner does not consent. She is totally fine but it is a little upsetting to watch.
Most of the movie could be considered claustrophobic. Being in an oil rig and nuclear sub under water with low ceilings and narrow halls and people drowning
Many drowned bodies are shown near the beginning of the movie. About midway through the rig begins to floor and many people are locked behind a door. It is implied that they drown. Near the end of the movie when two characters are trapped in a small underwater vehicle one chooses to drown and be later resuscitated. The scene is pretty upsetting as the water rises and they begin to panic and drown onscreen. At the end a character resigns to running out of oxygen but is saved at the last minute.