Spade Key
From Starfy Wiki
The Spade Key (Japanese: スペードキー) is blue key in Densetsu no Starfy. It's found in the Sunken Ship, originally under the possession of Boctopus. It is one of four keys required to open Sunken Ship's prison cell and rescue Dolchina and Dolshi. The other keys are the Heart Key, the Club Key and the Diamond Key. They are all based on the four French playing card suits.
It's found through the top-left door of the main room (with the prison cell). Through this door, Starfy must navigate a room of strong currents. Starfy can pass through the currents with the Mighty Star Spin, though there are taps to temporarily remove the currents when they are hit with a Star Spin so that the player can navigate the room more easily.