The offshore industry is rapidly contracting. Where there used to be perhaps 10 rigs, all requiring their own crew, there's now often just a single crewed rig (with all of the the others operated remotely from there), so there's very high unemployment in the industry.
Further, there's quite a bit of training which is necessary (such as ensuring that you can get out of an upturned helicopter which has been forced to ditch at sea, and advanced fire training) before you can even be allowed anywhere near to an oil rig. Most potential offshore workers have to pay for their own training (at a cost of several thousand pounds) but then find that, without experience, they still can't get a job in the industry.
Most cargo ships have very small crews these days and they're most commonly drawn from low-wage countries (such as the Philippines, India and China). So there are very few opportunities for people from the UK. (To get into the Merchant Navy these days you usually need specialist qualifications, such as degree in a relevant subject).
To be honest, your chances of finding a well-paid employed position in most fields are probably fairly slim. Your best chance of success is likely to be via self-employment but it must be up to you to decide which field offers you the greatest opportunities.