About 100 horses ran in 11 heats on the mudflats between the North Sea island of Neuwerk and the coastal town of Cuxhaven. A twelfth heat was cancelled due to rising tides after about six hours of racing.
The first of the injuries came shortly before the race as horses pulling a spectator wagon spooked. Four of the nine passengers sustained minor injuries, police said.
During the race a photographer was injured after being run over by a horse. Police are investigating and believe the animal had deviated from the racecourse, a spokesman said.
The woman was transported to the hospital and is believed to have sustained severe injuries, the spokesman said.
Locals ran the first of the annual Duhnen races on mudflats exposed by the tide in 1902. The event is known as the only horse race in the world run on the sea floor.
Winners of past year's races to dominate again in this year's event included Bernd Schrödl, of Magdeburg, who won two races, and Marisa Bock, who won the trot and took second place in the gallop with Glority.