You may differ of course, but how about I back up what I wrote with further information?
Your multiple entry tourist visa work restriction refers to the whole of the UK who’s National jurisdiction covers the UK mainland and extends to 12 miles offshore.
Once outside the 12 mile zone you are technically in international waters and therefore, under current UK legislation, do not require a work permit when working offshore in the North sea oil and gas industry. Because you are out-with the UK your tourist visa no longer applies and you can work your socks off.
Conversely any foreigner without a work permit. on shift on (say) a vessel working inside the 12 mile zone or alongside, is breaking UK immigration law. They could be arrested and suffer the penalties you suggest. The employing company (ROV operator/agency) could also be prosecuted. The employer might view it as an unnecessary expense and pay the fine. The job goes on. The foreigner on the other hand suffers far heavier long term consequences.
A foreigner intending to work offshore can state that fact on entry into the country. Immigration will allow them in on a tourist visa but will not give them permission to work offshore as there is no requirement to do so. They might remind the foreigner that, on a tourist visa, they are not allowed to work in the UK. That’s it.
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