From WiKirby, your independent source of Kirby knowledge.
Secret Sea is the seventh level in Kirby: Squeak Squad. It is presumably the ocean where Meta Knight's ship, the Halberd, ended up when it crashed in Revenge of Meta Knight and as such features it in stages along with many underwater sections. Additionally, a cloud-top section is also featured. In order to access this level, Kirby must, in addition to completing Ice Island, collect all five Star Seals in the previous areas up to this point. The boss of this stage is Meta Knight.
Each stage is unlocked in succession after clearing the previous one. Defeating Meta Knight is necessary to move on to Gamble Galaxy. For more information about a particular stage, click on the link to the stage's page in the table.