ArduPilot-4.7.0-beta1 has been released for beta testing including the Sub firmware. For this release we are planning to syncronise the Sub software with the other frames (Copter, Plane, Rover, etc) in the hopes that this reduces the administrative burden but also makes it easier to understand what is included in each release.
The list of changes can be seen in the ArduPilot Sub-4.7.0-beta1 release announcement.
In general we will be running the beta testing in the ArduPilot Sub-4.7 forums so if you find any issues, please create a new topic and include as much information as possible. Ideally include a log file if you can. FYI we track all the reported issues here.
BTW, this is a join effort with the BlueRobotics devs team (including @williangalvani) many of whom are also members of the ArduPilot dev team.