A new study looking at plastic pollution has concluded that most of it is coming from cruise liners, not land-based sources as has been commonly blamed.
The foregoing is not correct. Both reports refer to "ships" not "cruise liners".
While I am no fan of cruise ships, my (much) better half is and, consequently, I have been aboard a number of the Celebrity liners and, during such cruises, spent a lot of time going through the ships vitals with a number of the officers and engineering staff while my wife did the Spa thing. As I have been critical of waste from "ships", and particularly plastic water bottles, I was particularly interested in how the ships managed waste.
Aboard Celebrity, and I am reasonably sure other cruise lines as well, I found that trash is sorted, plastic bottles, glass and cans separated out, shredded or crushed and bailed/bagged for shore side recycling or disposal. As we did two cruises "back-to-back" on one occasion, we remained aboard during a lay-over and I watched from our stateroom balcony as bails of shredded plastic, compacted cans, and bags of crushed glass were unloaded to trash haulers on the dockside below. The ship's officers, and particularly the ship's Captain/Commanding Officer--a lady by the way--were all very impressively concerned about the quality of their waste management. I suspect other cruise lines have similar protocols but doubt that commercial cargo vessels come even remotely close and there are far more of those a sea.
One needs lay blame where it is due and not generalize from the specific in the reports. (FWIW I think disposable plastic drink bottles should be entirely banned but that's another discussion.)