Protesters plunged half-naked into the icy sea and unfurled banners Saturday to try to stop oil prospecting near Spain's Canary Islands, a major tourist destination.
Ten boats from the archipelago took protesters eight nautical miles from where Spanish firm Repsol is exploring with a view to possibly drilling off the islands in the Atlantic ocean.
Protesters warn the oil and gas project is a threat to the environment and the tourist industry on which the Canary Islands rely.
They say drilling would raise the risk of an oil spill like the Deepwater Horizon disaster that struck at a BP oil prospect in the Gulf of Mexico in 2010.
The government says finding oil could create thousands of jobs and reduce Spain's dependency on energy imports. The country currently imports 80 percent of its energy.
The beaches on the archipelago off northwest Africa are a popular draw for tourists from Britain, France and elsewhere.
Opponents of Repsol's operations are furious at the Spanish government for authorising Repsol to probe below the sea bed 50 kilometres (30 miles) from the islands of Lanzarote and Fuerteventura.
Environmentalists have branded it a threat to dolphins and other local fauna and flora.
"I have been a boat owner for 10 years and what they are doing here pains me. I am sick of seeing the sea polluted and destroyed," said Samuel Rocio Garcia, 32, a protester who dived into the water.
On board one of the boats was the leader of the local government from the island of Lanzarote, Pedro San Gines Gutierrez.
He said the protest was "a symbolic act of vigilance" to try to monitor the activities of the Rowan Renaissance, the ship Repsol is using to probe below the sea bed.
Spanish authorities last month temporarily impounded a boat of the environmental campaign group Greenpeace after it protested at the Repsol project in the same area.
Spain said the crew had defied orders to leave a restricted zone.
On November 15, three Spanish navy boats rammed vessels in which Greenpeace activists were approaching the Rowan Renaissance, a video distributed by Greenpeace showed.
An Italian protester fell in the water and was injured, Greenpeace said. It said its activists were protesting peacefully.