Ok so things have moved on…
The Nvidia Jetson Nano was released early 2019, making the Raspberry PI look a bit sad…I am currently porting all my python code that ran the web-based Raspberry PI ROV over to the Nano for increased performance - there is even NodeRed for the Nano available as a pre-built install. An updated illustration of the new architecture is below. Thanks to @earthling bx10 for the encouragement!
One very interesting video is the 3D mapping using the Jetson Nano and the Stereolabs ZED camera. These USB cameras tend to come with the IMU (i.e. heading, pitch, roll, etc) already integrated - handy!. It does assume a level of water clarity but possibly worth a trial.
