Deep Black is very simple to describe; it's Gears of War with an aquatic theme. Like its role model, the entire game is based around taking cover and popping out to take out enemies as they move. In order to make the game unique - and because this developer seems to only make water-based games - they've thrown in some mildly interesting swimming sections.
Other than the swimming, this game is just a lackluster, sloppy Gears of War. The graphics are pretty good and I especially liked some of the water effects, but the controls are decent at best and frustrating at worst. The gunplay works properly, but running is highly annoying since it's automatic and very finicky, and the cover system is simply disappointing if you've ever played Gears of War. Using cover works exactly the same, except without any of the finesse that made Epic's game so much fun to play. You can't roadie run from cover to cover, you can't jump over cover and so on.
What really lowers this game is the extremely dull combat situations and lack of flow that runs throughout the entire game. Every single combat situation is exactly the same. You turn a corner and five guys are camping behind cover, just waiting for you. Every time, your piece of cover is always a few feet too far ahead so you have to take damage while running up to it. Then you have to heal for a few seconds and then all you do is pop up, fire, wait, pop up, fire, wait. The very definition of repetetive.
They tried to throw in some bosses to mix it up, but they're so insanely stupid that they never pose any kind of a threat. The solution to all of them is to just circle a piece of cover and let them fire all their bullets into it as you fire yours into them. They also tried to include some variety by making some enemies rush at you like idiotic suicide bombers, but that's just annoying and at least partially ruins the illusion of the game. Are these enemy special ops soldiers trying to kill me or are they all on PCP and think I'm a gingerbread man that they should eat? It's just stupid.
Despite all these complaints, the game does come off as acceptable in hindsight. There wasn't much that actually frustrated or annoyed me, it was just consistently boring. Not so boring that you end up frustrated and unable to sit still, but it's never fun either. It doesn't matter how fancy their water engine is or how decent the controls are if the game isn't fun, and this game never poses an interesting challenge and is never fun.
(I would comment on multiplayer but this game sold so poorly that it was impossible to get a match going during prime time on release day.)