Do we need a visa for Sanya if we are on a cruise stop off for a day?

Hello. We are 2 british citizens who are boarding a Royal Caribbean cruise ship in Hong Kong (after a 2 night stay) on 10th October. We are cruising to Sanya in China (Hainan province) and we will be there for one day before setting sail for Vietnam and then Singapore. Do we need to apply for an entry permit for Sanya, as I am reading conflicting information? We haven't decided whether to book an organised excursion arranged by the ship or explore Sanya under our own steam. Thanks.