Things I remember about this book:
Humans had been forced by alien race(s) to either demilitarize or severely cut back on military to join galactic community
Unknown aliens attacking in Solar System
An expedition is launched on a reconditioned warship. I definitely remember that the ship had a distributed AI system, which humans were apparently very good at as compared to the other galactic civilizations
There was an FTL system in general use, the author has one of the characters highlight a long string of numbers as coordinates involved with this system
Coordinates where raiding aliens are originating from are discovered. Previously mentioned expedition goes through coordinates, lands at the bottom of an unidentified ocean
Possibly galactic representatives on expedition
Ending was definitely a cliffhanger, but I couldn't find a sequel at the time