Sphere, by Michael Crichton.

A psycologist is taken by the US Navy to the middle of the ocean and joined by several people he knows - people he picked out for a government team to deal with an "ULF" - unidentified life form. Things get weirder when they are told that there is a spacecraft at the bottom of the ocean. When they start to piece together where the craft came from, things get even weirder. Accidents start to occur in their deep-sea habitat, and people start to die.

The characters don't get too developed, but that's ok - most of them get killed off very soon. The plot is reasonable, but there are a few leaps that require a bit too much suspension of disbelief. And if you've already seen the movie when you read this, well, it's not going to leave you too happy - the movie adaptation takes the usual liberties, and it's just not the same.