Peggie Hall wrote:

You cleverly left out the operative phrase that I've stressed: "within
3 miles of the coastline"--which includes the coast of any islands.

You mean this statement?

"So unless you're only using the the head when you're out to sea at
least 3 miles from the nearest point on the whole US coastline, ..."

The donut holes are indeed formed by the radius of 3 mile distances off
the nearest points of land but a vessel in one is far from "out to sea."
That boat is sailing the coastal waters of Alaska in the INSIDE passage.
There are many places on the US coast that have distances between points
of land that exceed three miles but do not have donut holes. How wide is
the Chesapeake Bay? Puget sound and the Strait of Juan de Fuca have
areas more than 3 miles from land in several spots and do not fall into
that category, so it takes more than a simple 3 mile distance to define
the limit of no dumping.

The Alaska donut holes are a unique exception but they are an exception
and any blanket statement such as "there are no coastal (or inland)
waters in the whole US--east coast, west coast, or Gulf--in which it's
legal to discharge raw untreated toilet waste directly overboard or dump
a tank" is wrong.

Rick