The Staten Island Ferry's fare-free service is seen as both a critical transportation link that brings transit-starved residents to a paid subway ride and as an unfair freebie to its critics.

Now, there are calls to hit tourists with a $4 fare to fund transportation in the city from Queens advocates trying to keep ferry service in the Rockaways past October.

"Staten Island is already receiving a generous subsidy, while Rockaway is still desperately in need of permanent ferry service and better transportation options," Queens Assemblyman Phil Goldfeder said in a statement.

Just 15% of ferry riders in Staten Island live outside of the city, according to a recent Independent Budget Office analysis of the idea. The city paid an average of $5.75 a passenger this year. Charging for rides could generate $35.5 million in net revenue over 15 years with a stand-alone fare system; letting riders pay with a MetroCard would generate $67.4 million over the same period if the MTA paid the city for each swipe of an unlimited-ride.

Borough President James Oddo, who had requested an IBO analysis of tourist fares in 2006, has been cool to the idea.

Assemblyman Matthew Titone of Staten Island's north shore fiercely defended the free ferry, particularly as a way to encourage tourists to spend time and money on the borough once they enjoy seeing the sights from the boat.

He warned his colleague in Queens: "look but don't touch."

"A fee to tourists on the Staten Island Ferry is very extraordinarily ill-conceived and ill-advised," Titone said.

"I think it could impact tourists coming over as well as the money they would spend while here," he added.

Local opposition aside, there are logistical hang ups, such as changing the law to charge visitors and New Yorkers from outside Staten Island, tracking and verifying riders' address and paying for turnstiles, according to DOT spokesman Scott Gastel.