In an operation involving the Nation Crime Agency (NCA), the man, 49, and the woman, 53, were detained after officers discovered four tonnes of cannabis aboard their vessel.
The vessel was intercepted as it sailed through the Strait of Gibraltar on December 10.
The NCA had warned Spanish authorities that the boat in question may be involved in smuggling drugs from Morocco.
After escorting the vessel to the port of Cadiz, the authorities discovered 130 bales of cannabis.
The man and the woman remain in custody.
NCA international regional manager Steve Reynolds said: “This was an international operation which led to a substantial seizure of narcotics.
“We suspected the vessel was being used to ship drugs across the Atlantic, so the NCA worked closely with MAOC-N and our colleagues in Spain to have it intercepted.
“Working with our law enforcement partners we are determined to do all we can to target and disrupt the organised crime networks involved in the international drugs trade.”