Traditional dead reckoning is reasonably accurate over moderate timescales in the mostly empty ocean as subs are big and relatively slow. Thus the existing approach of only occasionally surfaces for GPS. This is therefore more a supplement as being able to regularly recenter even a few times a day is good enough.

I would assume that you'd use that data in conjunction with others (sensor fusion / kalman filter).

Wenn you know you were at location X 10 minutes ago and now one sensor tells you you are at Y ... You can reasonably assume that that sensor is wrong.

Doesn't matter just take into account what didn't move. you don't have to recalibrate at each instant.