Idea is to make side inspection with 2 cameras. If your camera is 24fps maximum speed is air around 15 cm/s. If speed is above 15cm/s when you place pause in video, stills will be blurry. In water this maximim ROV speed is around 10cm/s.
So if you have 120fps camera your maximum speed is 120/24 = 5x faster.
Max speed is 10cm/s x 5 = 50 cm/s = 0.5m/s. At this speed you will see all details which are bigger than 2mm.