How about an NVQ/SVQ in Hydraulics?
You have more than enough qualifications on the electrical side. Personally I would not waste valuable resources on Offshore survival, medical or health and safety courses. If you get a job you will be given all of those anyway, in my experience companies do not take these into account when choosing people as anyone they take on will be given them anyway.
For ROVs electrical, electronic and hydraulics are the fields you will need to know about, hence why I would suggest some hydraulic qualifications. You will not be designing circuits (electrical or hydraulic) so Degrees etc. do not count as much as more "practical" qualifications. You need to be able to maintain, identify faults and rectify. Practical experience and qualifications will always count more.
As for doing an ROV course, when you read through the site you will see that opinions vary on whether they are worthwhile or which couyrse suppliers are better/worse etc.