The sailor needs to row the boat to the island with redflag within one song time, and during the process, sailor will hear Siren is singing. In order not to be lured by the Siren’s voice, sailor needs to maintain the rhythm of rowing by hitting all the bubbles in the sea.
If the sailor follows the rhythm of Siren’s song while rowing, the tentacles controlled by Siren will appear to attack the boat. When 10 bubbles are missing, tentacles will completely drag the boat underwater.