This is the 1454th most frequent Japanese word.
石垣島
Ishigaki Island; an island in Okinawa, Japan.
In this sentence, '石垣島' is used as a proper noun referring to the geographical location, Ishigaki Island.
石垣島は沖縄県に属する美しい島です。
Ishigaki Island is a beautiful island that belongs to Okinawa Prefecture.
Here, '石垣島' indicates the intended destination of the visit explicitly in its use as a place name.
夏休みに石垣島を訪れる予定です。
I plan to visit Ishigaki Island during my summer vacation.
In this context, '石垣島' is used to specify the place whose sea possesses the described characteristic.