These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Dyeing for Your Art: The cast had to become certified divers before shooting began (as in being put through a course usually reserved for diving INSTRUCTORS). Shooting underwater extensively also required that they also undergo decompression before leaving the set. Ed Harris also sustained injuries on-set significant enough that he nearly had to turn down roles after shooting had wrapped. Not to mention that certain members of the crew, who literally spent close to the entire day underwater, for months, during the shoot, actually had their hair bleached white by the chlorine in the water.
Enforced Method Acting: One such scene led to Ed Harris decking Cameron after he almost drowned for real. Mary Elizabeth Mastrantonio got her own bit of that, too, with the resuscitation scene. Cameron kept demanding retakes, meaning she was repeatedly getting her chest pounded on(gently, but still) while soaking wet and half naked. She stormed off the set and wouldn't come back until Cameron agreed to wrap it already. And lest you think this was her being a prima donna, she stormed off specifically because they had actually run out of film stock shooting the many re-takes, and Cameron still wasn't happy.
Jerkass Woobie: Lt. Coffey wasn't exactly a nice guy to begin with, hell, he even mocks Lindsay when she tries to inform the troop about how High Pressure Nervous Syndrome, but there are times when it becomes clear how severe his condition is and he becomes a very pitiable man because of that.
Karma Houdini: It could be argued that the aliens fall into this, as they (probably accidentally) cause the crew of the nuclear sub to die, and are never called on it
"THE CRANE! WE LOST THE CRANE! IT'S ON ITS WAY DOWN TO YOU!"
Squick: Coffey cutting rank chevrons into his arm with a K-Bar.
Stop Helping Me!: When Lindsey is trying to get the rest of the crew to believe her about the underwater aliens, resident conspiracy nut Hippy goes into his usual Conspiracy Kitchen Sink speech. This results in a zinger: