Officers intercepted a speedboat carrying about 3,500 litres of petrol on the beach at Las Palmerillas in Roquetas de Mar, Almeria, at around 7am on Saturday.
Sources close to the case said the boat was fitted with two outboard engines and had a combined power of 700 horsepower. The fuel was being carried in jerry cans.
The crew was not found nearby, and the vessel was later towed away by a maritime patrol boat.
The seizure comes amid ongoing concern about the use of fast boats to support drug trafficking along the Almeria coast.
Boats used to supply drugs at sea
In recent months, police and civil guard units in the area have repeatedly warned about the scale of maritime logistics linked to narcotics runs, especially the transport of fuel for fast craft.
Local officers and police groups have also called for more resources on the coast, arguing that the problem has become increasingly visible in parts of the province.
Costa News Spain Breaking News | English News in Spain.