A cruise line operates luxury ships for entertainment purposes and people use their service for luxury and leisure travel. Huge ships that carry few thousand people generate millions of gallons of liquid waste and tons of solid (and hazardous) waste every month. Many of such cruise ships dispose off their waste illegally into ocean, away from shore. The waste includes, grey water (from sinks, galleys, etc.), toilet waste (or blackwater), oily water, hazardous waste, solid waste, etc.
By law, cruise ships cannot dispose off the waste near shore; however, there are no laws to prohibit the dumping of this waste in international waters. Cruise lines may resort to waste disposal in international waters to save on the cost of treatment and also the time it takes to offload the waste during the layover at shore. Since, there are no restrictions to disposal in international waters, the dumping of waste is not illegal.
Also note that given the heavy investment in this business, cruise companies resort to repeat customers and brand value, much more than many other businesses and hence may avoid such practices. However, those who do, would do so to save on the treatment cost and time required to offload the wastes (as already stated).
Hope this helps.