Usually, most underwater robots are running on just inertial navigation and surfacing occasionally. This tends to work better than one would expect and is what most large scale deployments end up using. Additionally, most of these use a Doppler Velocity Log, Pressure Sensor, and Compass in addition to their IMU.
Geophysical localization is fairly precise, ...
Can I use current sensing to measure the torque on a wormgear drive?
Nope as the worm-gear is not back-driveable by construction.
what would be the best path forward?
Using an FT sensor mounted at the end-effector would be the best solution but they are expensive.
Replacing the worm-gear with a BLDC motor being careful about the gear ratio. In fact, a ...