At sea is a correct place of birth. We see it in census records all the time.
A pregnant woman gives birth on a voyage from one country to another.
We also see death locations as at sea.
These should only be used when someone is in open international waters I would think.
That is my opinion. I do not know if the error report accepts at sea as a valid location, but it should be allowed or come up with something that covers when a child was born or person dies outside of any country, like those who died on the Titanic. They would have a death location of at sea. In the notes you could add aboard the Titanic.