In reply to Hans:
On the offshore side, as well as rig work, there's the possibility of survey vessel work.
You still need the BOSIET (offshore survival) course....done in Warsash, Fleedwood, Aberdeen, Hull and probably a couple of other places. Google it for dates and costings. And you'd still need an offshore medical too.
Several of the larger contractors run their own training for new hires. Mainly geophysicists, surveyors, engineers but open to most people with a "numerate" degree.
For non-technical people there are openings as Marine Mammal Observers. You need to do a training course first. There's a 3 day one at Warsash I believe. Most seismic/ survey vessels now routinely carry MMOs for operations in most parts of the world. I know several students who've done the course then financed their way through uni by spending their summers offshore...most have gone on to spend 2-3 years doing it full-time (usually 5 weeks on, 5 weeks off)to get a bit of financial security. It's freelance, generally, and rates to the MMO tend to be in the order of £300 per day. You spend a lot of time on bridge wings with binoculars!