I'm not saying they can come to an instant stop. But if they do stop they are in not in danger to themselves or the surrounding traffic. That means that if you see a sticky situation about to happen then you can start slowing down to make everything easier and you can keep slowing down until you are either able to safely handle it or completely still.
If the computer has no idea whats going on then slowing down and requesting human assistance is very viable.
Stopping in heavy weather or currents is not at all safe. The ocean is a dynamic environment. Vessels must maintain steerageway or else they risk broaching or drifting into obstacles. In shallow waters a vessel in distress can sometimes drop anchor to hold position but that isn't always a practical option.
If the computer has no idea whats going on then slowing down and requesting human assistance is very viable.