Startide Rising
Startide Rising picks up about 200 years after Sundiver left off. The first mostly-dolphin starship crew is assembled and sent on a survey mission. Upon finding a huge derelict fleet of unknown origin and sending a message home about it, several hostile races hound the small survey craft until she crashes in the ocean of a metal-rich waterworld.
Apparently, the hostiles are all quasi-fanatical races who believe in the eventual return of the Progenitors, the race or races who began the practice of Uplift. It soon becomes clear that the fanatics think the derelict fleet is a sign pertaining to the Progenitors, and they want the location of that fleet badly.