As I live in Tokyo, you need to fly to Okinawa, get a bus|taxi|walk to terminal in Naha, catch a ferry, another bus to beach. As I said if we are doing a comparison of parameters to grade, Zamami, it would get 3 zeros for cost, access and parking. Also the ferry leaves early morning so you will need to stay in a hotel in Naha the day your flight arrives and take the ferry the next morning. Hotels are limited on the island and I suggest you do a day trip. They also have a camp ground but I have never used it so please do your research on what to bring if you decide to camp.
“Zamami Island is an island in the Pacific Ocean. It is part of the Kerama Islands group and administered as the village of Zamami in Shimajiri District, Okinawa Prefecture, Japan. Zamami Island is 24 kilometers in circumference. The island has 3 settlements, which are Zamami, Ama, and Asa.” (citied from Wikipedia)