If we randomly assume they were halfway down (no idea on where they actually were but as a blind guess 50% is a good starting point)

The wreck was found 500m away from the wreck of the Titanic, the Titan descends in a curve and not straight downwards, gives pretty good indication that they were near the depth of the ocean floor. Combine that with the fact that they descended faster than anticipated and that they lost communication right around the time they were supposed to reach the lowet point, I think they were close to the ocean floor.

But cautiously saying half is probably better.