The revelation came as Europe's migrant crisis snowballed with Austrian police rescuing a group of dehydrated migrants found in a truck, days after the bodies of 71 migrants, believed to be Syrian, were found in an abandoned former poultry lorry.
"In August, 15 people were arrested for bringing Moroccan migrants to Spanish shores," the Guardia Civil, a paramilitary police force, said in a statement without specifying the nationality of those held.
In the last 72 hours alone "six water scooters were intercepted while depositing immigrants" off the coast of Tarifa on Spain's southernmost tip, which is separated from Morocco by only 14 kilometres (8 miles) of water.
Each water scooter brought over between two and three migrants across the Strait of Gibraltar and "posed a serious risk to the lives of immigrants ...who often don't know how to swim," the statement said.
Some people smugglers also bring in large quantities of hashish from Morocco, often weighing up to 30 kilogrammes, a source in the Guardia Civil told AFP.
One kilo of hashish from Morocco's Rif region has a street value of 1,600 euros ($1,800) in Spain.