I’m with Asturias on this one. I’ve been involved with gravity surveys, including airborne ones.
The main problem is extracting the signal from the noise. If you wish to acquire precision gravity data, the first thing you do is keep the gravimeter stationary.
For a typical oil exploration gravity survey, you are looking for a precision of about one milligal. This is roughly 1 millionth of a G!
If you are bouncing the gravimeter around in an aircraft, even on a relatively calm day, the processing required is extremely difficult and one of the things you sacrifice is the ability to detect small scale features.
In the case of a survey I was involved with, we were able to locate the margins of a buried sedimentary basin that was tens of kilometres across. We were not expecting to be able to find oil-bearing structures.
I haven’t done any calculations, but I suspect that the gravity signal from a submarine, even near the surface, would be buried in the noise, regardless of the sophistication of the measuring equipment.