Reader, this is X-Com: Enemy Unknown/UFO Defense with an underwater coat of paint, and the difficulty dialed to 11+.
It does have an amazing unsettling, and oppressive atmosphere, but it comes at so much cost to balance.
In the 40 or so years since the end of the First Alien War, technology has stagnated a bit. Elerium was in limited supply, and wasn't waterproof, so X-Com was reduced to scavenging the seafloor for what little they can recover.
That means starting from square 1, so what cool new weapons do you start with? Dart guns, canister rifles, and harpoon guns. "Eat darts, ya varmints!" just doesn't have the same ring to it.
Nearly every technology has been turned into the proverbial foam dart in some way. The deck is stacked, and the missions are much longer than before. If you thought the bug hunts of before were grueling, you'll be happy to know that Terror From the Deep introduces two-part marathon missions.
Terror from the Deep is more X-Com. It is more X-Com, but harder.