Basically, “autonomous” means independent, not controlled from an “outside” source. We generally think of the tether as outside control, so ditching the tether and carrying out an underwater ROV mission, regardless of how simple, is in fact, an autonomous operation. There are many situations where GPS is not a requirement for autonomous operation.
For example, if I program my ROV to go to the bottom (not worrying about current), video record for a specified time, then return to the surface, that is an autonomous mission. This is a simple example.
Next up is to program in a heading to run on for a given time on the way down, etc. More advanced is to take into account the current, know how rapidly the ROV will descend, then send the ROV to a point in the underwater space, autonomously.
I haven’t taken the risk to do this out by our reefs yet, but it is certainly in the future. Using the GPS in the boat and noting the drift velocity and direction on the surface (boat movement), I can estimate fairly closely a heading , forward RPM, and descent RPM which will get me to the bottom where I want to be.
Just something to think about. We reached the moon through dead reckoning and understanding the movements of the earth and moon, and the effects of gravity. We can do the same thing in the ocean. Super easy in a swimming pool or lake.