Some poop and sewage does end up in the ocean even from sewage treatment.
In some places sewage is sometimes dumped into the ocean even when it's not fully treated.
However most cruise ships at least don't dump sewage into the ocean and instead empties the tanks on board once the get back to port.
It still is possible though that some other ships may dump their sewage into the ocean.
A recent modeling study finds that wastewater adds around 6.2 million tons of nitrogen to coastal waters worldwide per year, contributing significantly to harmful algal blooms, eutrophication and ocean dead zones.
In most cases of sewage being dumped into the ocean chemicals are added to kill as many germs as possible.
Then the treated water is released into a local river or even the ocean.
If you live near the coast your treated sewage probably goes into the ocean.
The treated sewage is cleaned to make sure that it does not cause environmental problems.
The sewage produced on board navy ships cannot be held on the navy ship for an extended time.
For this reason the sewage on board a navy ship must be discharged into the sea.