[BitWielder1]

There are many, many more oil tankers and cargo ships than there are cruise ships that even if all cruise ships went 100% green, the planet would not notice.

[SJackson]

even if all cruise ships went 100% green, the planet would not notice.

True, actually they're as green as a Tesla or Chevy Volt, bellweathers of the anti carbon set. Most cruise ships have electric propulsion units, like the aforementioned autos. Carbon energy sources are needed to produce electricity, neither extension cords of adequate length nor batteries of sufficient storage capacity not yet existing. For those greenies for whom 100% electric propulsion is the goal, cruise lines are essentially there.

[entropy12]

BitWielder1 hits the nail, with how many cargo ships and personal boats are there, most more that 25 years old.

What ALL the ships could use however is wind turbines to operate when wind direction is favorable (more in direction of travel). There is usually lots of wind away from land.

Also why not throw in some solar panels as well. Most cruise ships operate in sunny climates.