It's fine to dump sewage overboard if you're far enough from shore. Cruise ships treat it to some extent, but ultimately it all becomes fish food.
In the same way that every breath you take contains at least one atom-or-molecule of Caesar's last breath, every sip you take contains at least one atom-or-molecule of every cruise ship waste jettison. (Assuming sufficient worldwide diffusion time.)
It seems that in most ports they are required to hold and treat it (and, perhaps to a lesser extent, in inshore waters.) Fouling the places they take their customers to is ultimately bad for business.