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For years, cruise lines have vied with each other to offer the most restaurants at sea, the scariest waterslides, the most breathtaking feature (you know, the one where you gasp, ‘You can do that on a cruise ship, really?’). Now there is a new contest – the one to be the greenest of them all.

Driven by changing public attitudes and tougher environmental regulations, companies are investing billions in new technologies that will keep our oceans and atmosphere as clean as they can be.

At its simplest, it’s about banning single-use plastic as much as possible; at its most complex, it has meant developing engines that run on alternative fuels (liquefied natural gas is the current favourite, said to reduce sulphur emissions by