A great sequal in a great serie.
It sticks wonderfully to the theme of pop-cult alien nasties: where we had martians from Cydonia in the first game, we have ancient Lovecraftian horrors lurking beneath the waves. Also, finally we don't have to reequip every soldier after every mission anymore, thank god for that.
Alright, so it's not that well thought through that you can throw grenades around underwater, but I'd like to say "duh" to reviewers who complain about the X-Com troops not being able to swim: they are wearing armored-up heavy diving suits, that's why they can walk on the sea-bed at all.
So it's basically more of the same with the difficulty scaled up, in a marine environment and with some additional tactical doohickeys up it's sleeve, but if you've finished UFO Defense, and you don't want more of the same but harder, there's something wrong with you.