From the cover:
"Unknown to anyone, two creatures have wandered the Earth for generations. The aliens have no knowledge of each other, but share a residual memory of a mysterious, sunken relic - and an affinity for deep water. One, the changeling, has survived by adaptation, taking the shapes of many different organisms. The other, the chameleon, has suvived solely by destroying anything or anyone that threatens it.
Now, finally brought up from the bottom of the sea by marine biologist Russel Sutton, the relic calls to them both... to come home. For all these generations there have been two invincible creatures on Earth. But the chameleon has decided there's only room for one..."

Das Buch ist Nebula-Preisträger 2006

If this was really the best science-fiction-novel 2006, then 2006 was not a good science-fiction-year... The novel is not bad, but also not really good.
In the first part, I only liked the chapters dealing with the story of the changeling (itself says at one point, that it regrets, not to have come out of the ocean a few ceturies earlier - and as a reader I have to agree: THAT would have been an interesting story). The parts set in the near future (actually: now - I read the chapter set on December 14th 2019 on December 14th 2019), are kind of boring: some scientists research on a strange artefact that does not reveal anything. The only interest I had in these parts was trying to figure out who of the persons was the alien.
When in the second half the two timelines meet it gets more interesting, especially the changeling's flight from the CIA. - But the love story was quite annoying, though the concept of fluid gender is kind of interesting.
The final confrontation between changeling and chameleon is a little bit short. - Otherwise: from the story point of view it is never a good idea to confront two super-human, immortal enemies...

Waits in the book shelf for new readers