Sewage carries bacteria and viruses that are harmful to humans and it also can sicken and kill marine life, including corals.
It carries the pollutants nitrogen and phosphorus, which contribute to algae blooms that cloud the water and lead to reduced oxygen levels that can kill fish.
Oil, which is also released from cruise ships, is a highly toxic substance that does tremendous damage to marine life.
Finally, marine debris, or trash, strangles, starves and even kills marine animals such as sea turtles and birds. Plastic bags, for example, can be mistaken for food and eaten by sea turtles. The plastic fills their stomachs and causes difficulty with digestion and loss of appetite. Since it provides no nourishment, plastic debris can lead to starvation.
No systematic studies have been done to demonstrate that cruise ship sewage is safe for marine life. We do know that sewage harms marine life, including coral reefs, regardless of where it comes from.