While doing a little research on what might affect local gravity the most, I came across some interesting facts:
The sea floor crust is mostly 10km thick.
Sea thickness is never more than around 10km
So that means seafloor+sea is never more than 20km total thickness.
Continents never rise more than 9km above the top of the sea (Everest at 8.8km.)
So if the bottom of the continental crust were to match the bottom of the sea floor crust, continental crust couldn’t exceed 29km, but it does in many places. This means that continental crust descents deeper into the mantle ellipsoid of the earth than does the seafloor crust. The crust of the Himalayan mountain range is 70km thick, and it rises no more than 9km over the top of the sea. That means roughly 50km of that crust resides under the bottom of sea floor crust! So as 2 plates smash together, much more buckles under than over sea level, before melting back into the mantle.
I suppose continents very slowly plow through seafloor crust similar to an icebreaker ship breaking through sheets of ice, resulting in earthquakes and volcanic activity. Instead of floating on water they float on mantle. Mid ocean ridges being where resulting cracks are quickly filled in with magma coming up then cooling into new crust.
Some density values (higher the density the lower you go)
1027 kg/m^3 average ocean density
2850 kg/m^3 average crust density (I’ve read sea floor crust is slightly less dense than continental)
3300 kg/m^3 average mantle density
While the crust of the Earth is no more than 1% of the Earth’s total mass, it’s the closest to surface, so it might have enough of a gravity effect to noticeably alter the flow of water due to density variations. The mantle might not be so uniform in density either but the effects of mantle variations would be less due to the distance.
3D gravity data
While searching for information on vector gravimeters I came across these links
It would appear that the vast majority of data is taken from at least hundreds if not thousands of feet in the air. If these measurements were made in conjunction with surface (and perhaps even sub surface) measurements, one might be able to create a decent 3D gravity map.
This vector gravity data could be fed to a computer program to determine what configuration of say a few dozen mass points with any mass and any location (in 3D space) would most closely cause the resultant gravity vector data at each measurement point.
This 3D gravity map might allow a sort of GPS in underground tunnels. Satellite GPS won’t work underground as the ground absorbs the Electro Magnetic waves. One could correlate changes in the gravity vector while traveling in a tunnel to a 3D gravity map to determine exactly where they are in the tunnel.