Came across this article on the use of accelerometers for navigation without GPS. A long way to go in development but would revolutionize navigation in underwater, cave and GPS dead zones.
This is a very interesting article and technology. It sounds like the biggest hurdle to overcome to bring this to autonomous vehicles including ROVs is to find a replacement for the bio mechanical solution that is used to correct the false motion in the IMU. Maybe there would be a way to use multiple IMUs to determine when the ROV is perfectly flat for an instant or something like that.
Another thought I have had and maybe it exists is a “smart tether” that can output 3d data on its shape and length underwater from the ROV to the surface unit. The surface unit would have a GPS receiver. A correction could be done later in post that places the photo capture location accurately using the 3d shape of the tether and the GPS.
The smart tether is a cool idea, there was another accelerometer based solution, but it is still way too large for commercial use.