I got a grade E in A level Physics many moons ago, then went to University and got a degree in Electronic Communication Engineering.....Then after working for 4 years for the MoD I went offshore to work on survey vessels, not as a seaman (fnar fnar) but as a computer geek and have been for the past 8 years. We do 6 weeks on, 6 weeks off, and I personnaly wouldn't do any more time offshore than 6 months out of a year. I hate the job now, its !!!!!e, but its a means to an end, and the benefits are that you don't have to pay tax
My brother went into the merchant Navy at 18, did all the cadet stuff, and now at 36 is a Captain of a dive support vessel working out of Trinidad, and gets paid a wad. The thing to do is get a bridge ticket, then get a DPO (Dynamic positioning officer) course under your belt, then freelance as a DPO on a day rate. Typical day rates are around £200/day.
Working on a ship has its moments, like when you're stood on deck at 6am watching the sun come up on a beautiful morning (or rather when you're in 84kts of wind in the North Sea in the winter, and the waves are peaking at 15m; ~50 feet and a high pressure fuel leak occours in the engine room
).
Good luck
EA