Japanese sisters, 26 year old Chinatsu Yamada and 33 year old Natsuki Yamada, were reported missing on Monday when they failed to turn up for their scheduled departure.
The police think the sisters may have drowned when swimming at night.
Our correspondent, Mark Rabago, says 60 people are involved in the search, which includes rescue boats, jet skis and divers.
He says the search has now shifted from Saipan's Wing Beach, where the sisters were staying and their car found, to Tanapag Lagoon, after the United States Coast Guard used a device that simulates a human body adrift at sea.
"They think the waters may have drifted the sisters there if they drowned. The police stated they have ruled out foul play on the disappearance of the sisters and are still trying to move heaven and earth to locate them."
Mark Rabago says fixed-wing aircraft and helicopters from the United States Navy will be used if need be.