Boat sewage can spread disease
Raw or poorly treated sewage in the water can spread bacterial and viral diseases. Human waste in marine waters can also spread Norovirus (formerly called Norwalk), which causes a severe and highly contagious intestinal illness. Norovirus can survive for months in salt water and become concentrated inside shellfish such as oysters, clams and mussels, especially during warm weather.
Boat sewage can close beaches and shellfish beds
Sewage in the water can force the closure of swimming beaches and recreational and commercial shellfish beds. The Washington State Department of Health has periodically closed commercial shellfish beds near marinas and mooring fields because of risks from boat sewage.
Boat sewage can harm wildlife
When sewage breaks down, it uses up oxygen in the water and releases carbon dioxide, which can harm fish, shellfish and other wildlife. This is a particular concern in shallow water and areas of low tidal flushing.