Camouflage — Joe Haldeman

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 survived solely by destroying anything or anyone that threatens it.

Now, finally brought up from the bottom of the sea by marine biologist Russell 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...