They seem to have little doubt that the film took its premise from the play.
There’s no specific mention of plans to take legal action against The Shape of Water, though if the Paul Zindel estate is certain the idea was taken without credit, that would seem to be the next logical step.
Fox Searchlight vigorously defends Guillermo del Toro and claims the writer and director of The Shape of Water has never read nor seen the play in question, but according to The Guardian, the similarities between the two stories have been remarked on by people familiar with both, and now the family of the playwright has weighed in on the discussion.