Emergency services were called to Hornsea at about 18:00 BST.
Crews, including Hornsea Inshore Rescue, HM Coastguard and Humberside Police, remain at the scene. The RNLI said its Bridlington all-weather lifeboat joined the search.
Ch Supt Phil Booker said: "Our priority now is to find the boy, and members of the public can expect to see specialist Police Search Advisors, along with our Marine Unit and drone pilots, who have also been deployed to conduct an extensive search and rescue operation to locate him."
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