He numbers the waves.
The source for the proverb is English Proverbs and Proverbial Phrases by William Carew Hazlitt (Google Books). The poster is made with AutoMotivator. The image is from Wikipedia.
Counting the waves is proverbial fool's errand. There's also an Aesop's fable about counting the waves which features a fox who adds quite a wise moral to the story: Aesop tells this story. A certain man was sitting on the beach counting the waves as they broke against the shore. When he lost count of the waves he got angry and frustrated. A sly fox then approached the man and said, 'Good sir, what is the point of getting angry about the waves that have already gone by? You need to just put them out of your mind and pick up counting again where you left off!'