Spade Key
From Starfy Wiki
The Spade Key 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 Sunken Ship. In this room, Starfy must navigate a room of strong currents. Though Starfy can pass through them with the Mighty Star Spin, there are taps to remove the currents so that the player can navigate the room more easily.