what sort of coax is it? I’m not sure if it’s possible to boost the signal.
Do you have any other conductors in the cable assembly? I only ask, because you could control the ROV (without video) over the spare conductors (Control will work fine over standard power cables, no need to be coax), and run a separate analog camera (composite or HD- AHD/TVI) over the coax-you’d need an additional display to watch the feed though. Quality of the feed will depend on what conductors you have and what camera you choose, but these formats should be ok over 200m (composite for sure, HD-AHD/TVI maybe not).
Let me know if you need coax with additional conductors as I may be able to help.