NAMPA, Idaho—Justin Harrison is fighting for the right to surf in Idaho.

This type of surfing needs no ocean. It is called wakesurfing, and it lets someone surf, untethered, in a speedboat’s wake as though riding a briny wave. Boats designed for wakesurfing are hot sellers at Idaho Water Sports, the shop Mr. Harrison manages, and elsewhere. On a recent day on Lake Lowell, he drove a boat while employee William Nunnelee followed 10 feet behind, carving the wake on his surfboard.

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