"The members of the organisation picked up the drugs in Moroccan waters hiding them in false bottoms built in the pleasure boats," police said in a statement.
"This way the group managed to hide to the plain eye, both on land or by air, the transported drugs," it added.
Police said the group would transport the drugs from Moroccan waters to the coast of the southern province of Cadiz from where they would be taken by boat on the Guadalete River to remote warehouses.
They seized five pleasure boats with false bottoms and 4.5 tonnes of hashish in several raids carried out since April 2015. They also arrested 17 suspected members of the gang, including its leader.
Spain's proximity to North Africa, a key source of hashish, and its close ties with former colonies in Latin America, the world's main cocaine-producing region, have made it a gateway into Europe for drug consignments.
Spanish police seized around 100 tonnes of hashish last year, about three percent less than in the previous year.