E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The ninth wave. A notion prevails that the waves keep increasing in regular series till the maximum arrives, and then the series begins again. No doubt when two waves coalesce they form a large one, but this does not occur at fixed intervals. The most common theory is that the tenth wave is the largest, but Tennyson says the ninth.
And then the two
Dropt to the cove, and watchd the great sea fall,