Cruise Lines for many years have earned a bad wrap for their non-sustainable practices. Historically they have been known to hurt ecosystems by introducing invasive species through their ballast water (just realized the college oceanography project was not a waste of time lol). Socially cruise ships can leave destination ports trashed and mainly benefit a few local businesses.
However, I will give credit where it is due. Cruise Lines International Association (CLIA) is what I consider to be the governing body for cruise lines. Because maritime regulation is so wacky due to boarders and open waters the cruise lines turn to CLIA as a way to communicate with one another. In December 2018 CLIA announced a historic global cruise industry commitment to reduce the rate of carbon emissions across the industry fleet by 40 percent by 2030 (read more). This was a BIG win for environmental sustainability. CLIA was able to get the industry as a whole to agree on this initiative. Since then many cruise line operators have worked on there own accords to make environmental, social, and economic changes that reduce harm.
One example of a cruise line making a difference is MSC. MSC is using pop culture as a tool and taking advantage of the upcoming super bowl. They have started the MSC Foundation which has the vision to restore the critical balance between people and nature within a generation.