TBH mate,
It doesnt matter… you choose what you want to do based on the info you research. It can only be your choice after all. HNC in Electronics/Electrical is all en-compasing for that discipline, ditto for mechanical. I would suggest you look at what you would like to do after ROVs, as ROV wont or might not last forever. Do you really want to do offshore work till your 60-70? (as some guys do).
There are also better jobs out there so be careful about what you choose as it will cost you some time and money…
Good luck!