The ocean would be most likely (subterranean would be much more visible for any access points, by satellite and land-mapping LIDAR, or seismographs detecting underground industrial activity). But we get detritus washing up all the time on shores; for example, Japanese debris from the earthquake landed on western North American shores.

How does an aquatic super-civilization exist for hundreds (presumably thousands) of years with absolutely zero debris arriving on shores, or any evidence of mining or other industrial activity that would be required?

Interstellar seems more likely but of course we have the challenges of time and distance to explain.