That means there are nearly 500 cruise ships burning heavy marine oil.
There are alternatives to burning heavy marine oil, the industry just chooses not to take them up. There is no real alternative to JET-A1.
Left as an exercise for the student:
'How many A380s flying 100 miles does it take to produce the same amount of CO2, NOx, SO2, microparticulates and soot as it does to equal the emissions of one cruise liner at dock for an hour?'
Far better to take a jet and spend longer at your destination than to burn dirty fuel getting there.