Back to the alligators in the sewers: That seems to have originated with a book by Robert Daley called "The World Beneath The City", published in 1959 by Lippincott, about all the weird and interesting stuff below ground in NYC. It's a delightful book, and it includes an account by a veteran sewer inspector (the superintendent at the time) who describes seeing alligators about 2' long. The book states that they were all exterminated within a few months of their sighting; the outbreak is said to have occurred in 1935.

Peace. Paul