Trying to remember name and author of story where main character gets turned into a frog or something and is underwater. And the crayfish are knights and it is like a society. Gets yanked back out of the stream to normality before the battles are over.
I think I figured it out, myself. It is Land of Unreason by American writers Fletcher Pratt and L. Sprague de Camp.
Fred Barber, an American staying as a guest in an English country home during World War II, consumes a bowl of milk left as an offering for the fairies, substituting liquor in its place. The rightful recipient of the offering, drunk and offended at the substitution, takes vengeance by kidnapping Barber off to the Land of Faerie as a changeling, a fate normally reserved for infants. He finds Faerie beset by a menace echoing the war in his own world. Trapped in a magical realm where rationality as he knows it is turned upside-down and failure to follow the rules can have dire consequences, Barber undertakes a quest in the service of Oberon, the fairy king, in order to be returned to his own world. The outcome, befitting a realm in which nothing is as expected, is one that neither he nor the reader anticipates, for Fred Barber is not quite the man he thinks he is.
It indeed has crayfish knights, as per text in the Internet Archive copy, which can be borrowed.
The crawfish-knight was actually puffing in his indignation and Barber judged it prudent to change the subject. "You know," he said, "I'm on rather a serious mission as it happens. I'm looking for a piece of property that belongs to the Mother of the Gods, and I've been led to believe it might be in Whoozis' possession. Could you suggest how I might go about recovering it?"