Like watching a nightmare.
They don't make 'em like this anymore. Underwater is a claustrophobic and relentlessly fast-paced creature-feature that feels like it's been ripped out of 1989. The action starts at about 3 minutes in and doesn't relent until the credits roll. Characters are given as little backstory as possible, by and large their only goal is to not die.
I expect others will criticize the film for its anemic approach to story, but for me it felt like a breath of fresh…