First you need to pass 3 level of diving courses. There are written and practical test. You will need to learn hand signage, gears, safety, evacuation and most importantly how to combine N2 and O2 (only in level 3). Underwater, oxygen is very corrosive. Diver breath in more N2 and less O2 as they go deeper. There is this cumulative effect of saturated N2 in bloodstream which cause health problem at later age (forget the details  ).

Then you need to go for welding course, then only underwater welding. For info, welding isn't just about skill, it is also an art, not everybody can do it. You need to perfect your welding skills before going for underwater welding. In O&G, client can't afford defective welding, especially underwater. You will be out as soon as you get nt if you can't perform.

As for underwater inspector, you need to have a handy of experience in underwater work (7-10 years) before you are qualify.

Since you are from marine, my advice is to stay on. Once you promoted to chief engineer position, you get 5 figure easily, more stable job than underwater, more lasting career, less health risk, and overall better money. 
It's not that difficult to pass the survival test and get the offshore pasport la~ Provided you are not afraid of water and height 

Yeah, you can join O&G,but you need to start all over again when you join operation. You will get high salary only when you reach platform supervisor, manager status (depends on company though). Only those who are professionl and rare, i.e. turbine, engine, generator, compressor, system engineer will get those figures.

Get your sailing experience with tankers, cargo ship. Need to sacrifice few years of your youth though.

After a 5-10 years, join service company that provide troubleshoot, overhaul service to vessel's engine. Build your reputation, then you can go further by joining luxurious cruise ship, join 3rd party service company and become inspector, surveyor, etc for engine. You be travelling from port to port, usually within our region. Your income is either daily, per job basis, or hourly, depends on your experience and knowledge. Annual income can reach 0.5 million (with 15-20 yrs experience, by then you would probably be 40+)

heh found someone also from marine background here. so wonder are u all still sailing or stop already?

m a marine engineer too, now on break after 6 months working on gas carrier, quite making some big bucks now time to spend all and relax xD

what batch are u in? lol xD

anyone got MMHE HR email here? where to find their contact email? google didnt help    got their email in uni, but lost now

its not only about shipping la..MMHE also is the fabricator of Jacket & Topside (Platform) and several other offshore structure

If anyone is interested in getting into O&G ( as a fresh grad but the community prefer experienced ppl) do let me know what line u're in, be it HSE, ENG, etc. I get an average of 3-5 e-mails a week from friends and companies requesting me to recommend ppl for contract jobs, so if u're really interested to work short term, i.e. 3-12 months project let me know. Especially Mariners as they're in great demand.