The Merman's Children.
Can "alter" be a noun? Yes:
one of the distinct identities or personality states manifested in an individual with multiple personality disorder
-copied from here.
However, that meaning does not fit this context:
"They called up warning of a rock, and alter of a drifting boat, perhaps broken loose from a ship that was towing it." (p. 47)
I have been concentrating on terminology, language, prose and descriptions rather than on the plot because these are the aspects of a text that sf fans might not notice so much. Appreciating Poul Anderson on every level is quite hard work. I think that one of the magazines told me that they did not publish literary criticism - although I do not remember which one.
I will shortly go out for part of the evening so there will be a temporary halt to reading and posting although I think that we have had quite a lot for today. I have stayed indoors and taken it easy at least in a physical sense.