The short and true answer is that if I would know for sure I could not write about here. Hence all of this is speculation. Very likely they would use the best

The short and true answer is that if I would know for sure I could not write about here. Hence all of this is speculation. Very likely they would use the best motion isolation money can buy. Also the submarine is quite slow (especially at navigation speeds) so the terrain changes under it slowly. Water currents and the motion of people and machinary happens at much higher rate, so you can probably try to eliminate the quick changes and keep the slower changes with filtering and statistics.